Of the million or so types of microbes that exist, only 1,415 are known to manufacture human malady. Pathogens typically produce symptoms as a side effect of their lifecycle.
Each step of an infection is used to advantage by the invader. Pathogenic microbes assist each other to reproduce and spread: a cooperation honed over many generations.
Genetic mutations ratchet up during invasion. This alters the interface a pathogen presents to a host, raising the odds of a successful infection.
In an intricate life cycle of different lifestyles, many successful microbes ferry themselves between hosts using living vectors. To obtain optimal probability of distribution, those vectors often fly. Mosquitoes and birds have been common carriers of pathogens. The West Nile virus has managed worldwide distribution using both mosquitoes and birds as vectors.
Over time, evolution tends to mitigate devastation of a host. Virulence, in killing the victim outright, is disadvantageous, as the microbe is left homeless and is likely to die alongside/inside its host. With centuries of coexistence, there is the natural evolutionary tendency to optimize survival of both species, though the rapid adaptability of microbes generally puts them one step ahead of host response.
The carrier of a vector-based disease acts as passage but does not suffer the disease. The other route to human malady to is more direct and demonstrates another aspect of cleverness by microbes.
Corpuscular capitalists that they are, pathogens are constantly seeking new avenues for expansion. They rapidly adapting to exploit those opportunities. For a pathogen on the prowl, genic information from potential host cells allows a tweak to their own genes that makes a new host available for infestation.
Humans encourage many maladies by lifestyle choice, notably diet. Eating animals has been a particularly good formula for breeding disease and has successfully promoted cross-species contagion.
Humans give ambitious pathogens ample opportunity for entry by raising and slaughtering animals for food. Dipping one’s hands into the guts of an animal for meal preparation does wonders for maleficent microbes.
Animals have been the primary source of introducing infectious illness: 60% of human pathogens derive from animal contact, particularly mammals. Among new microbial maladies, the trouble transfer rate is ~75%.
Listeriosis is a most virulent foodborne illness, with a human kill-rate reaching 30%: around 500 deaths in the US per year. Listeria monocytogenes is the responsible bacteria. L. monocytogenes is commonly ingested along with dairy or other animal products, mostly raw, but including cooked poultry and smoked fish. The bacteria only rarely come from consuming raw vegetables.
Once inside, L. monocytogenes hijacks host cellular functions: subverting a body’s heterochromatin machinery and reprogramming the transcription of genes normally employed in innate immunity. The bacteria manipulate epigenetic regulation to wreak havoc.
Researchers spot a novel microbial nuisance about every 4 months. While most are trivial, a telling minority, such as HIV, avian flu, and SARS, pose grave threats.
Livestock cultivation promotes new vectors for pathogen victims. Congo fever comes from a tick-borne virus which is spread by many animals, including some birds as well as cattle. Incubation takes 1 to 3 days from a tick bite, and almost a week for exposure from the blood or tissue of someone infected. The malady made its way from Africa, where it was endemic to herding tribes, into the Middle East and elsewhere. With rising incomes comes opportunities for transmission. Importation of cattle into India has introduced Congo fever.
As people get a taste of prosperity via urbanization, they eat more animals. Prosperity is relative. Burgeoning slums are an ideal place to share disease.
Livestock not only pass new maladies to man and other species; they also help spread existing pathogens into new locales. While the market system shows no inclination to share the wealth, its practice is a paradigm for sharing ill health.
Sickness can result from an unhealthy microbiome, which practically invites infection by a badass bacterium. Researchers are only beginning to understand how changes in the microbiome inspire illness.
Chronic diseases are a dual phenomenon: hostile bacteria taking up residence alongside friends that have largely abandoned the neighborhood, or, worse, never moved in. Many diseases show a significant shift in the inner ecosystem. Not only is the onset of disease an invasion, it is an invitation for pathogenic low life to join the party. People with asthma have a different microbiome than healthy breathers.
Digestive disorders are disorder in the gut microbiome. Diabetes is directly related to the state of the gut microbiome.
Microbiome transfer can be beneficial. Gut infections have been successfully treated with fecal transplant: inserting feces from a healthy person into the colon of a sick person. That is a shotgun approach that offers only temporary relief unless the recipient changes dietary habits.