Jason Tetro in Popular Science:
Normally, when a pathogen enters the body, an able immunity is capable of defeating most invasions. B-cells, T-cells, macrophages, neutrophils and others work in combination with one another to eliminate the threat and restore us back to health. It’s not always an easy process, however, and at times can lead to exhaustion. When this happens, our ability to fend off other invaders decreases. We essentially become more susceptible to other ailments. One of the most common causes of exhaustion happens with chronic viral infection, such as human immunodeficiency virus (HIV) or hepatitis C virus (HCV). Both of these pathogens have the ability to evade the defenses and maintain a presence for years. As this happens, the immune system continues to fight without any progress towards victory.
A vital immune cell in the fight against viral infection is called the CD8 T-cell. It has the task of finding these tiny pathogens and purging them from the body. The cell accomplishes this by either signaling an infected cell to kill the virus or, more viciously, to kill the infected cell altogether. Depending on the type of signal they receive their population can be controlled such that the response is just right for the situation. Once an infection is cleared, most of the cells die but some are kept around to serve as memory for any future viral attacks. Unfortunately, when a chronic infection happens to occur, this process is interrupted and some of the CD8 T-cells lose their ability to function. This is known as exhaustion and can cause significant detriment against the current infection and worse, any new ones that happen to arrive. In HIV and HCV positive individuals, exhaustion is a serious concern as it may be the basis for even greater susceptibility to secondary infections.
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