[3] Ewald asserts, along with a growing body of studies, that many common diseases of unknown origin are likely the result of chronic low-level infections from viruses, bacteria or protozoa.
[7] Ewald argues that many common diseases of currently unknown etiology, such as cancers, heart attacks, stroke and Alzheimer's, may likewise be also caused by chronic low-level microbial infection.
Ewald, whose background is in evolutionary biology, points out that any disease causing gene that reduces survival and reproduction would normally eliminate itself over a number of generations.
The only benefit to the pathogen causing the sickness would be the potential transmission to other hosts; much like the particulate expelled during coughing, diarrhea can be a means of distribution.
Another major influence on Ewald's thinking in evolutionary biology terms was the HIV virus, which once caught, initially remains inactive for years thus allowing it to spread, before the chronic disease of AIDS finally manifests, incapacitates, and eventually kills the host.