To my understanding, the terminology "anaerobic" is used to describe an environment that is lacking in oxygen.
There are several species of microbes that thrive in such an environment, often causing severe, if not fatal, diseases. One such microbe is the clostridium botulinum, which causes a disease called botulism. Another microbe, clostridium tetani, causes the disease tetanus.
Both of these bacterium use what is called an "endospore" to help them survive and metabolize inside our bodies. "Endo" means "within," and "spore" means "seed." Simply put, an endospore is a tiny compartment inside the bacterium that usually lies dormant, but which causes the bacterium to grow and flourish, causing infection when placed in an anaerobic environment.
The best way to prevent these horrific diseases is to maintain the strictest cleanliness, cook food properly and thoroughly, maintain a healthy immune system, and in the case of tetanus--get regular tetanus vaccinations.