Definition

A pathogen is a germ, any tiny living thing or particle that can make you sick, including some bacteria, all disease-causing viruses, and some fungi and protists, though only a small fraction of microbes are actually pathogens, which is why washing your hands works by removing the few harmful ones along with the dirt. More formally, a pathogen is a biological agent that causes disease in its host, spreading through direct contact, contaminated food or water, the air, or vectors such as mosquitoes and ticks, with the body defending itself through barriers like skin and mucus and the immune system, while public health measures such as sanitation, vaccination, and antibiotics, effective against bacteria but not viruses, reduce infectious disease. At the mechanistic level, pathogenicity depends on virulence factors such as adhesins, toxins, capsules, and immune-evasion mechanisms, and on host immune status, with Koch's postulates setting the classic standard for linking a microbe to a disease and epidemiology quantifying transmission with the basic reproduction number $R_0$, the average number of secondary infections from one case, so the herd immunity threshold is approximately $1 - 1/R_0$.