What is a Secondary Infection?

MRSA bacteria in a petri dish

A secondary infection can occur when a different infection, known as the primary infection, makes a person more susceptible to disease. It is called a secondary infection because it occurs either after or because of another infection. In other words, it is secondary to that infection. 

A primary infection can increase susceptibility to disease in several ways. It can alter the effectiveness of the immune system. It can also make 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. They occur because the body is no longer able to fight off bacteria or viruses that a healthy immune system can normally repel. 

Skin infections that result from scratching molloscum contagiosum or similar sores are also secondary infections. These show how one disease can make it easier for another to get into the body. The sore from the STD makes it easier for other bacteria to enter and infect the skin. When someone scratches the sore, the damaged skin is easy for new bacteria to infect. In addition, scratching sores can spread an infection from one part of the skin to another. However, this type of spread is not considered a secondary infection. It's just an expanded version of the initial, primary infection. 

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. Antibiotics disrupt the normal vaginal flora. Those are the bacteria that are present in the healthy vagina. When they are gone, it gives  yeast, which are normally present at low levels, an opportunity to overgrow. That is why so many women end up with yeast infections after they've been given antibiotics. The antibiotics kill off the good bacteria in the body as well as the bad bacteria. Then other organisms, such as yeast, can seize the opportunity to multiply without competition. 

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. These are not always considered to be secondary infections.However, they are sometimes referred to in that way. 

The Difference Between Secondary Infection and Co-infection

Secondary infections occur after, or because of, primary infections. However, sometimes people have multiple infections at the same time that aren't directly related to one another. These infections are often considered to be co-infections rather than secondary infections. For example, people can be co-infected with both gonorrhea and syphilis. Those infections aren't necessarily related to each other. Instead, they're both related to similar types of activity - unprotected sex. In contrast, if people become infected with an oral yeast infection because of HIV--related immune suppression, that's a different story. The yeast infection is only possible because of the HIV infection. Therefore, it would be considered to be a secondary infection or opportunistic infection.  


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