When we identify a new pathogen, we are in the middle of the story. Only when the adverse affects, permanent debilitating condition or death reach a high enough threshold do we search for the source and cure. It may take a long time.
Nature does not come with tidy little flow charts linking pathogen and source.
Smallpox only exists in laboratories, as far as is known, because humans were the only reservoir, and the smallpox virus has been eradicated from nature. Smallpox was transmitted directly from human to human. There was no other reservoir and the last outbreak was caused by a laboratory accident, roughly 38 years ago.
Some pathogens follow a more complicated pathway. The reservoirs are not always known. Mosquitoes are one of humanities favorite vectors for pathogen transmission. So once the mosquitoes infect some people, the infected people may become a source for new infections.
Programmers like drawing simple diagrams to describe pluggable systems that would handle/hide all the complexity. So depending on the pathogen there may be a reservoir, there may be one or more vectors, or the vector may be identical to the population of humans as in the case of smallpox.
This view of the process is mostly missing pieces or fill in the blanks.
“For a long time most stomach ulcers and gastritis was attributed to stress(psychological/social causes) or spicy food(environmental cause)” inferred from this article on Helicobacter Pylori. A bacterial cause was demonstrated by Dr. Barry Marshall and Dr. Robin Warren of Perth, Australia in 1982. The bacteria may have been seen as early as 1875 and there were scores of near misses until 1982.
Slow to catch on and I don’t know if all stomach ulcers are accounted for by Helicpbacter Pylori.
Oh well, my big hollow mental model tires me out.