A pathogen is any agent that can make a host sick
From Greek roots for suffering and producer, pathogen entered English in the 1880s for disease-causing organisms and agents—viruses, bacteria, protozoa, prions, viroids, fungi, and sometimes larger parasites. Not every illness is infectious: coal dust, sickle cell genes, and lupus prove the point.
Pathogenicity mixes infectivity (ability to infect) with virulence (severity). Koch’s postulates still frame how microbes are tied to diseases. Transmission runs through bodily fluids, droplets, contaminated surfaces, or living vectors such as mosquitoes; soil can harbor threats longest. Worms and insects can also spread illness, but they are usually filed as parasites and studied by parasitology, while microbiology handles the microscopic agents. Some diseases have many possible culprits: meningitis can come from bacteria, viruses, fungi or parasites, whereas cholera arises only from certain strains of Vibrio cholerae. Opportunists strike mainly people with weakened immunity, often in hospitals.
Virulence strategies include stealing host nutrients, making toxins, and suppressing immunity. Optimal-virulence theory imagines a trade-off between spreading to new hosts and not killing current ones too fast. Reproduction numbers estimate secondary cases from one infection in a susceptible population.
Even odd cases fit the broad map: chlorophyll-free Prototheca algae can cause protothecosis in people and animals.
Bacteria range from 0.15 to 700 micrometres, and most are harmless or helpful, like the gut microbes that aid digestion. The dangerous minority rely on tools for sticking to and entering cells, capsules that dodge being engulfed, toxins, and siderophores that scavenge iron. Tuberculosis, mainly caused by Mycobacterium tuberculosis, carries one of the heaviest burdens, claiming 1.6 million lives in 2021, the majority of them in Africa and Southeast Asia. Pneumonia and food poisoning add many more bacterial culprits to the list.
Source: Pathogen
