By: Kristi Waterworth
No matter how closely you listen to your plants, you’ll never hear a single “Achoo!” from the garden, even if they’re infected with viruses or bacteria. Although plants express these infections differently from humans, some gardeners worry about plant disease transmission to humans — after all, we can get viruses and bacteria, too, right?
Although it would seem like a no brainer to assume that plant and human diseases are distinct and cannot crossover from plant to gardener, this isn’t the case at all. Human infection from plants is very rare, but it does happen. The primary pathogen of concern is a bacteria known as Pseudomonas aeruginosa, which causes a type of soft rot in plants.
P. aeruginosa infections in humans can invade nearly any tissue in the human body, provided they are already weakened. Symptoms vary widely, from urinary tract infections to dermatitis, gastrointestinal infections and even systemic illness. To make matters worse, this bacterium is becoming increasingly antibiotic resistant in institutional settings.
But wait! Before you run to the garden with a can of Lysol, be aware that even in severely ill, hospitalized patients, the infection rate of P. aeruginosa is only 0.4 percent, making it highly unlikely that you will ever develop an infection even if you have open wounds that come in contact with infected plant tissues. Normally-functioning human immune systems make human infection from plants highly improbable.
Unlike bacteria that can function in a more opportunistic fashion, viruses need very exacting conditions to spread. Even if you eat fruits from your squash mosaic infected melons, you won’t contract the virus responsible for this disease (Note: eating fruits from virus-infected plants isn’t recommended — they’re not usually very tasty but won’t hurt you.).
You should always cull virus-infected plants as soon as you realize they’re present in your garden, since they are often vectored from sick plants to healthy ones by sap-sucking insects. Now you can dive in, pruners blazin’, confident that there’s not a significant connection between plant diseases and humans.
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Associate Professor, Biomedical Sciences, Bond University
Associate Professor Lotti Tajouri is affiliated with Dubai Police Scientist Council.
Bond University provides funding as a member of The Conversation AU.
The Conversation UK receives funding from these organisations
Viruses are the most common biological entities on Earth. Experts estimate there are around 10,000,000,000,000,000,000,000,000,000,000 of them, and if they were all lined up they would stretch from one side of the galaxy to the other.
You can think of them as nature’s own nanotechnology: molecular machines with sizes on the nanometre scale, equipped to invade the cells of other organisms and hijack them to reproduce themselves. While the great majority are harmless to humans, some can make you sick and some can even be deadly.
(Photo: DAVID HECKER/AFP/Getty Images)
What is the evolutionary advantage to viruses of making us ill? originally appeared on Quora: the place to gain and share knowledge, empowering people to learn from others and better understand the world.
Answer by Suzanne Sadedin, Ph.D. in evolutionary biology, on Quora:
As a general rule, viruses don’t benefit from making us ill. They would much prefer* to keep us healthy. What benefits the virus is making copies of itself and spreading those copies to new human hosts.
When you get sick, you tend to stay home. You don’t move around much. You look sick, which means other people are less inclined to get close to you. From the perspective of the virus, all of this is thoroughly annoying: how is it supposed to go forth and conquer new hosts, when you’re sniveling in bed?
But the poor virus faces a trade-off. In order to infect new hosts, it has to create numerous copies. To create copies, it has to insert itself into your cells and co-opt their machinery for itself. Then those copies have to get out of the cells which created them, which usually means destroying the cells, which means you need to make more cells. This can sometimes damage your organs. But in most cases, sickness isn’t actually a direct result of cells being destroyed by viruses.
Your cells resent being taken over by viruses. They alert the immune system, which responds by raising your body temperature (viruses hate that, as their replication mechanisms work better at lower temperatures), and creating inflammation to destroy viral particles and infected cells. Your body also expels viral particles through any convenient orifice, which is a mixed blessing for the virus — it helps it spread to new hosts, while making it less likely to survive inside you. It’s usually this interaction between the immune system, the virus, and infected tissues that creates the symptoms of viral infection, which you experience as being sick.
The faster a virus replicates, the sooner it will attract the wrath of the immune system and lose its current host. But the slower it replicates, the less it can infect new hosts! Viruses solve this quandary in a wide range of ways. Some, like the common cold, are easy-come-easy-go infecting specifically the upper respiratory tract, where you can easily be coughed and sneezed out, and reproduce quickly there. Colds, in effect, assume that the immune system will soon kick them out, and make no serious attempt to survive in one host for more than a week or so.
Then there are some viruses, like Ebola, that make you really sick, really fast. Unlike the common cold, these viruses actively infect a wide range of tissues their strategy is basically to turn you into a gigantic leaky bag of virions, as quickly as possible, and hope that you drip on somebody else. It actually isn’t a very good strategy, especially in the modern world, since people tend to avoid fluid exchange with Ebola patients.
At the other extreme are sexually transmitted infections (STIs). If you look at things from the perspective of a genital herpes virion, you really want your host to be in top-notch shape. You want that host to be energetically pursuing and making love to new partners as often as humanly possible. If they have oozing sores all over their genitalia, that’s unhelpful.
So herpes and other well-established STIs tend to be very slow-growing, which allows them to be modest and discreet in their symptoms for years on end — all out of consideration for their hosts’ sexual exploits. (Interestingly, syphilis has evolved within historical time to become less aggressive in this way early accounts describe it as a rapidly progressing and horrendously disfiguring disease, whereas today many syphilitics go for years without even knowing they are infected.)
Many STIs also have a less benign strategy to prolong your love life they actively suppress the signalling systems that would alert the immune system to trigger inflammation and eradicate diseased cells. The immune system isn’t just useful for killing infections it’s also important to stop cells evolving to become cancerous. So this is how, for example, we get associations between HPV, cervical cancer, oral sex and throat cancer.
*Viruses and cells don’t actually have preferences, thoughts or feelings. It’s a metaphor.
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When it comes to birds, there may be more than just avian flu to be worried about. It has been suggested that there are over 60 other diseases that birds and their droppings can carry. The problem is especially worrisome in residential areas, as many of them are airborne and can be transferred to humans just by being around droppings.
Chicken farms can be fairly easily secured. With a combination of visual scare devices, sonic distress call emitters, ultrasonic disrupters and roost inhibitors other birds shouldn't be a problem. If farmers just took this preventative action it could help contain the bird flu outbreak a good deal.
Examples of transmissible bird diseases associated with pigeons, geese, starling and house sparrows:
Besides being direct carriers of disease, nuisance birds are frequently associated with over 50 kinds of ectoparasites, which can work their way throughout structures to infest and bite humans. About two-thirds of these pests may be detrimental to the general health and well-being of humans and domestic animals. The rest are considered nuisance or incidental pests.
A few examples of ectoparasites include:
Pathogenicity is the potential disease-causing capacity of pathogens. Pathogenicity is related to virulence in meaning, but some authorities have come to distinguish it as a qualitative term, whereas the latter is quantitative. By this standard, an organism may be said to be pathogenic or non-pathogenic in a particular context, but not "more pathogenic" than another. Such comparisons are described instead in terms of relative virulence. Pathogenicity is also distinct from the transmissibility of a virus, which quantifies the risk of infection. 
A pathogen may be described in terms of its ability to produce toxins, enter tissue, colonize, hijack nutrients, and its ability to immunosuppress the host.
It is common to speak of an entire species of bacteria as pathogenic when it is identified as the cause of a disease (cf. Koch's postulates). However, the modern view is that pathogenicity depends on the microbial ecosystem as a whole. A bacterium may participate in opportunistic infections in immunocompromised hosts, acquire virulence factors by plasmid infection, become transferred to a different site within the host, or respond to changes in the overall numbers of other bacteria present. For example, infection of mesenteric lymph glands of mice with Yersinia can clear the way for continuing infection of these sites by Lactobacillus, possibly by a mechanism of "immunological scarring". 
Virulence (the tendency of a pathogen to reduce a host's fitness) evolves when a pathogen can spread from a diseased host, despite the host becoming debilitated. Horizontal transmission occurs between hosts of the same species, in contrast to vertical transmission, which tends to evolve toward symbiosis (after a period of high morbidity and mortality in the population) by linking the pathogen's evolutionary success to the evolutionary success of the host organism. Evolutionary biology proposes that many pathogens evolve an optimal virulence at which the fitness gained by increased replication rates is balanced by trade-offs in reduced transmission, but the exact mechanisms underlying these relationships remain controversial. 
Transmission of pathogens occurs through many different routes, including airborne, direct or indirect contact, sexual contact, through blood, breast milk, or other body fluids, and through the fecal-oral route.
Algae are single-celled eukaryotes that are generally non-pathogenic although pathogenic varieties do exist. Protothecosis is a disease found in dogs, cats, cattle, and humans caused by a type of green alga known as prototheca that lacks chlorophyll.  Often found in soil and sewage, the species Prototheca wickerhami is the cause for most human cases of the rare infection of Protothecosis.  
The vast majority of bacteria, which can range between 0.15 and 700 μM in length,  are harmless or beneficial to humans. However, a relatively small list of pathogenic bacteria can cause infectious diseases. Pathogenic bacteria have several ways that they can cause disease. They can either directly affect the cells of their host, produce endotoxins that damage the cells of their host, or cause a strong enough immune response that the host cells are damaged.
One of the bacterial diseases with the highest disease burden is tuberculosis, caused by the bacterium Mycobacterium tuberculosis, which killed 1.5 million people in 2013, mostly in sub-Saharan Africa.  Pathogenic bacteria contribute to other globally significant diseases, such as pneumonia, which can be caused by bacteria such as Streptococcus and Pseudomonas, and foodborne illnesses, which can be caused by bacteria such as Shigella, Campylobacter, and Salmonella. Pathogenic bacteria also cause infections such as tetanus, typhoid fever, diphtheria, syphilis, and leprosy.
Fungi are eukaryotic organisms that can function as pathogens. There are approximately 300 known fungi that are pathogenic to humans  including Candida albicans, which is the most common cause of thrush, and Cryptococcus neoformans, which can cause a severe form of meningitis. The typical fungal spore size is 
Prions are misfolded proteins that are transmissible and can influence abnormal folding of normal proteins in the brain. They do not contain any DNA or RNA and cannot replicate other than to convert already existing normal proteins to the misfolded state. These abnormally folded proteins are found characteristically in many neurodegenerative diseases as they aggregate the central nervous system and create plaques that damages the tissue structure. This essentially creates "holes" in the tissue. It has been found that prions transmit three ways: obtained, familial, and sporadic. It has also been found that plants play the role of vector for prions. There are eight different diseases that affect mammals that are caused by prions such as scrapie, bovine spongiform encephalopathy (mad cow disease) and Feline spongiform encephalopathy (FSE). There are also ten diseases that affect humans such as, Creutzfeldt–Jakob disease (CJD).  and Fatal familial insomnia (FFI).
Not to be confused with Virusoid or Virus. Viroids are the smallest infectious pathogens known. They are composed solely of a short strand of circular, single-stranded RNA that has no protein coating. All known viroids are inhabitants of higher plants, and most cause diseases, whose respective economic importance on humans vary widely.
Viruses are small particles, typically between 20 and 300 nanometers in length,  containing RNA or DNA. Viruses require a host cell to replicate. Some of the diseases that are caused by viral pathogens include smallpox, influenza, mumps, measles, chickenpox, ebola, HIV, rubella, and COVID-19.
Pathogenic viruses are mainly from the families: Adenoviridae, Coronaviridae, Picornaviridae, Herpesviridae, Hepadnaviridae, Flaviviridae, Retroviridae, Orthomyxoviridae, Paramyxoviridae, Papovaviridae, Polyomavirus, Rhabdoviridae, and Togaviridae. HIV is a notable member of the family Retroviridae which affected 37.9 million people across the world in 2018. 
Protozoans are single-celled eukaryotes that feed on microorganisms and organic tissues. Considered as "one-celled animal" as they have animal like behaviors such as motility, predation, and a lack of a cell wall. Many protozoan pathogens are considered human parasites as they cause a variety of diseases such as: malaria, amoebiasis, giardiasis, toxoplasmosis, cryptosporidiosis, trichomoniasis, Chagas disease, leishmaniasis, African trypanosomiasis (sleeping sickness), Acanthamoeba keratitis, and primary amoebic meningoencephalitis (naegleriasis).
Parasitic worms (Helminths) are macroparasites that can be seen by the naked eye. Worms live and feed in their living host, receiving nourishment and shelter while affecting the host's way of digesting nutrients. They also manipulate the host's immune system by secreting immunomodulatory products  which allows them to live in their host for years. Many parasitic worms are more commonly intestinal that are soil-transmitted and infect the digestive tract other parasitic worms are found in the host's blood vessels. Parasitic worms living in the host can cause weakness and even lead to many diseases. Parasitic worms can cause many diseases to both humans and animals. Helminthiasis (worm infection), Ascariasis, and enterobiasis (pinworm infection) are few that are caused by various parasitic worms.
Although bacteria can be pathogens themselves, they can also be infected by pathogens. Bacteriophages are viruses, also known as phage, that infect bacteria often leading to the death of the bacteria that was infected. Common bacteriophage include T7 and Lamda phage.  There are bacteriophages that infect every kind of bacteria including both gram-negative and gram-positive.  Even pathogenic bacteria that infect other species, including humans, can be infected with a phage.
Plants can play host to a wide range of pathogen types including viruses, bacteria, fungi, nematodes, and even other plants.  Notable plant viruses include the Papaya ringspot virus which has caused millions of dollars of damage to farmers in Hawaii and Southeast Asia,  and the Tobacco mosaic virus which caused scientist Martinus Beijerinck to coin the term "virus" in 1898.  Bacterial plant pathogens are also a serious problem causing leaf spots, blights, and rots in many plant species.  The top two bacterial pathogens for plants are Pseudomonas syringae and Ralstonia solanacearum which cause leaf browning and other issues in potatoes, tomatoes, and bananas. 
Fungi are another major pathogen type for plants. They can cause a wide variety of issues such as shorter plant height, growths or pits on tree trunks, root or seed rot, and leaf spots.  Common and serious plant fungi include the rice blast fungus, Dutch elm disease, chestnut blight and the black knot and brown rot diseases of cherries, plums, and peaches. It is estimated that pathogenic fungi alone cause up to a 65% reduction in crop yield. 
Overall, plants have a wide array of pathogens and it has been estimated that only 3% of the disease caused by plant pathogens can be managed. 
Animals often get infected with many of the same or similar pathogens as humans including prions, viruses, bacteria, and fungi. While wild animals often get illnesses, the larger danger is for livestock animals. It is estimated that in rural settings, 90% or more of livestock deaths can be attributed to pathogens.   The prion disease bovine spongiform encephalopathy, commonly known as Mad cow disease, is one of the few prion diseases that affect animals.  Other animal diseases include a variety of immunodeficiency disorders that are caused by viruses related to the Human immunodeficiency virus (HIV) including BIV and FIV. 
Humans can be infected with many types of pathogens including prions, viruses, bacteria, and fungi. Viruses and bacteria that infect humans can cause symptoms such as sneezing, coughing, fever, vomiting, and even lead to death. Some of these symptoms are caused by the virus itself, while others are caused by the immune system of the infected person. 
Despite many attempts, to date no therapy has been shown to halt the progression of prion diseases. 
A variety of prevention and treatment options exist for some viral pathogens. Vaccines are one common and effective preventive measure against a variety of viral pathogens.  Vaccines prime the immune system of the host, so that when the potential host encounters the virus in the wild, the immune system can defend against infection quickly. Vaccines exist for viruses such as the measles, mumps, and rubella viruses and the influenza virus.  Some viruses such as HIV, dengue, and chikungunya do not have vaccines available. 
Treatment of viral infections often involves treating the symptoms of the infection rather than providing any medication that affects the viral pathogen itself.   Treating the symptoms of a viral infection gives the host immune system time to develop antibodies against the viral pathogen which will then clear the infection. In some cases, treatment against the virus is necessary. One example of this is HIV where antiretroviral therapy, also known as ART or HAART, is needed to prevent immune cell loss and the progression into AIDS. 
Much like viral pathogens, infection by certain bacterial pathogens can be prevented via vaccines.  Vaccines against bacterial pathogens include the anthrax vaccine and the pneumococcal vaccine. Many other bacterial pathogens lack vaccines as a preventive measure, but infection by these bacteria can often be treated or prevented with antibiotics. Common antibiotics include amoxicillin, ciprofloxacin, and doxycycline. Each antibiotic has different bacteria that it is effective against and has different mechanisms to kill that bacteria. For example, doxycycline inhibits the synthesis of new proteins in both gram-negative and gram-positive bacteria which leads to the death of the affected bacteria. 
Due in part to over-prescribing antibiotics in circumstances where they are not needed, some bacterial pathogens have developed antibiotic resistance and are becoming hard to treat with classical antibiotics.  A genetically distinct strain of Staphylococcus aureus called MRSA is one example of a bacterial pathogen that is difficult to treat with common antibiotics. A report released in 2013 by the Center for Disease Control (CDC) estimated that each year in the United States, at least 2 million people get an antibiotic-resistant bacterial infection, and at least 23,000 people die from those infections. 
Due to their indispensability in Bacteria, essential persistent DNA methyltransferases are potential targets for the development of epigenetic inhibitors capable of, for example, enhance the therapeutic activity of antimicrobials,  or decrease a pathogen's virulence. 
Infection by fungal pathogens is treated with anti-fungal medication. Fungal infections such as athlete's foot, jock itch, and ringworm are infections of the skin and can be treated with topical anti-fungal medications like Clotrimazole.  Other common fungal infections include infections by the yeast strain Candida albicans. Candida can cause infections of the mouth or throat, commonly referred to as thrush, or it can cause vaginal infections. These internal infections can either be treated with anti-fungal creams or with oral medication. Common anti-fungal drugs for internal infections include the Echinocandin family of drugs and Fluconazole. 
Algae are commonly not thought of as pathogens, but the genus Prototheca is known to cause disease in humans.   Treatment for this kind of infection is currently under investigation and there is no consistency in clinical treatment. 
Many pathogens are capable of sexual interaction. Among pathogenic bacteria, sexual interaction occurs between cells of the same species by the process of natural genetic transformation. Transformation involves the transfer of DNA from a donor cell to a recipient cell and the integration of the donor DNA into the recipient genome by recombination. Examples of bacterial pathogens capable of natural transformation are Helicobacter pylori, Haemophilus influenzae, Legionella pneumophila, Neisseria gonorrhoeae and Streptococcus pneumoniae. 
Eukaryotic pathogens are often capable of sexual interaction by a process involving meiosis and syngamy. Meiosis involves the intimate pairing of homologous chromosomes and recombination between them. Examples of eukaryotic pathogens capable of sex include the protozoan parasites Plasmodium falciparum, Toxoplasma gondii, Trypanosoma brucei, Giardia intestinalis, and the fungi Aspergillus fumigatus, Candida albicans and Cryptococcus neoformans. 
Viruses may also undergo sexual interaction when two or more viral genomes enter the same host cell. This process involves pairing of homologous genomes and recombination between them by a process referred to as multiplicity reactivation. Examples of viruses that undergo this process are herpes simplex virus, human immunodeficiency virus, and vaccinia virus. 
The sexual processes in bacteria, microbial eukaryotes, and viruses all involve recombination between homologous genomes that appears to facilitate the repair of genomic damage to the pathogens caused by the defenses of their respective target hosts. 
Familiarity with the way plant diseases are visually identified can help you diagnose problems.
Most plant diseases – around 85 percent – are caused by fungal or fungal-like organisms. However, other serious diseases of food and feed crops are caused by viral and bacterial organisms. Certain nematodes also cause plant disease. Some plant diseases are classified as “abiotic,” or diseases that are non-infectious and include damage from air pollution, nutritional deficiencies or toxicities, and grow under less than optimal conditions. For now, we’ll look at diseases caused by the three main pathogenic microbes: fungus, bacteria and virus. If plant disease is suspected, careful attention to plant appearance can give a good clue regarding the type of pathogen involved.
A sign of plant disease is physical evidence of the pathogen. For example, fungal fruiting bodies are a sign of disease. When you look at powdery mildew on a lilac leaf, you’re actually looking at the parasitic fungal disease organism itself (Microsphaera alni). Bacterial canker of stone fruits causes gummosis, a bacterial exudate emerging from the cankers. The thick, liquid exudate is primarily composed of bacteria and is a sign of the disease, although the canker itself is composed of plant tissue and is a symptom.
A symptom of plant disease is a visible effect of disease on the plant. Symptoms may include a detectable change in color, shape or function of the plant as it responds to the pathogen. Leaf wilting is a typical symptom of verticilium wilt, caused by the fungal plant pathogens Verticillium albo-atrum and V. dahliae. Common bacterial blight symptoms include brown, necrotic lesions surrounded by a bright yellow halo at the leaf margin or interior of the leaf on bean plants. You are not actually seeing the disease pathogen, but rather a symptom that is being caused by the pathogen.
Here are a few examples of common signs and symptoms of fungal, bacterial and viral plant diseases:
Bacterial disease signs (difficult to observe, but can include):
Bacterial disease symptoms:
You can see that there is a lot of overlap between fungal, bacterial and viral disease symptoms. Also, abiotic diseases, herbicide injury and nematode problems must be considered possibilities when an unknown plant problem appears. These lists are not complete or exhaustive, only examples.
Michigan State University Extension offers publications and online information to assist producers in identifying and controlling serious plant diseases. In addition, MSU Diagnostic Services offers online factsheets covering many common plant diseases in Michigan, and can diagnose diseased plant samples at an affordable cost. The lab website has submittal forms and details on sample submission and costs.
For more basic information on plant disease, visit Ohio State University’s Introduction to Plant Disease Series webpage.
This article was published by Michigan State University Extension. For more information, visit https://extension.msu.edu. To have a digest of information delivered straight to your email inbox, visit https://extension.msu.edu/newsletters. To contact an expert in your area, visit https://extension.msu.edu/experts, or call 888-MSUE4MI (888-678-3464).
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