TLRs tend to prompt the immune system to respond to infectious agents like bacteria and viruses - not allergens.

BBC: Immune system 'fights allergies'

Giving a computer the ability to fight off infectious agents sounds odd, but it makes a lot of sense.

ECONOMIST: A thousand ills require a thousand cures

These sites work as switches activating one biochemical pathway when they encounter the chemical signals of infectious agents, and a different one when they meet with those of an interloper organ.

ECONOMIST: Transplant surgery

Epidemiology has a long list of accomplishments, including most famously identification of the association of smoking with risk of lung cancer, but also encompassing the identification of many other exposures, medications, and infectious agents as risk factors or protective factors for specific diseases.

FORBES: Does Coffee Drinking Really Protect Against Devastating Diseases?

First, most natural forms of biological agents are not highly infectious, and highly infectious seed stock is difficult to obtain.

ECONOMIST: Letters