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.
Organisms that don't need free oxygen to survive are called anaerobic. Some types of bacteria and other single-celled organisms can live without oxygen, but nothing as complex had been found until 2010 as three species of Loricifera which have been dicovered in oxygen free water in the deepest parts of the ocean. The evolutionary history of these creatures is not yet known, but they live in an environment reminiscent of Earth’s oceans some 600 million years ago, before the deep seas were oxygenated and large animals evolved.
Look further in :
“The first metazoa living in permanently anoxic conditions.” By Roberto Danovaro, Antonio Dell’Anno, Antonio Pusceddu, Cristina Gambi, Iben Heiner and Reinhardt Mobjerg Kristensen. BMC Biology, Vol. 8 No. 32, April 6, 2010.
“Anaerobic animals from an ancient, anoxic ecological niche.” By Marek Mentel and William Martin. BMC Biology, Vol. 8 No. 32, April 6, 2010