A primary infection can increase susceptibility to disease by altering the effectiveness of the immune system or by making it easier for the secondary infection to get into the body. The opportunistic infections associated with AIDS are a good example of the types of secondary infections that occur when a disease modifies the immune response.
Skin infections that result from scratching molloscum contagiosum or similar sores show how one disease can make it easier for another to get into the body.
Treatment for a primary infection can also lead to secondary infections. One common example of this is how antibiotic treatment leaves women more susceptible to yeast infections. Individuals may also experience infections at the insertion sites of IVs, catheters, and other types of treatment that leave foreign objects in the body for extended periods of time.