In our bodies, we carry many germs – such as bacteria, fungi, and viruses. When our immune system is working, it controls these germs. But when the immune system is weakened by HIV disease or by some medications, these germs can get out of control and cause health problems. Infections that take advantage of weakness in the immune defences are called “opportunistic”. People who are living with HIV are particularly susceptible to opportunistic infections unless they have a healthy CD 4 count and low viral load, or else on HIV medication.