Mycobacterium avium complex (MAC) is a type of bacteria that can cause disease by infecting humans with weakened immune systems (the system in the body which helps to fight off various infections). HIV/AIDS is a common cause of a weakened immune system that may predispose someone to MAC infection. MAC is found in food, soil, water, and is usually harmless to people with healthy immune systems. Symptoms include fever, abdominal pain, fatigue, diarrhea, and low blood counts (anemia). Since the MAC bacteria are in contact with humans so often, it is hard to prevent exposure; however, there are available preventative medications for people with weakened immune systems.