What Is a Secondary Infection?

A secondary infection is one that occurs when a different infection, known as a primary infection, has made a person more susceptible to disease. It is called a secondary infection because it occurs either after or because of another infection.

Why Secondary Infections Occur

There are several ways that a primary infection can increase susceptibility to disease, leading to a secondary infection.

Changes to the Immune System

Some diseases can lower the immune system's ability to fight off harmful invaders like viruses and bacteria. This can make it easier for them to enter and cause a secondary infection.

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 can no longer fight off bacteria or viruses that a healthy immune system usually can.

Compromised Skin

Skin infections can compromise the skin's ability to act as a barrier to the outside world, particularly when they also cause breaks or sores. These can act as entry points for new infections.

For example, when someone scratches a sore from a sexually transmitted infection (STI) such as molloscum contagiosum, it spreads the infection from one part of the skin to another, worsening the primary condition. But it can also make it easier for other bacteria to enter and infect the skin, causing a secondary infection.

Consequence of Treatment

Treatment for a primary infection can also lead to secondary infections. One common example is how antibiotic treatment leaves people with more vaginas more susceptible to yeast infections.

Antibiotics are helpful when treating bacterial infections because they target and kill bacteria. However, in doing so, they kill off both the bad and the good—including normal, healthy vaginal bacteria (flora).

Though the antibiotics may have resolved the primary infection they were intended to treat, the loss of vaginal flora that results means other organisms, such as yeast, can seize the opportunity and multiply without interference. 

Individuals may also experience infections at the insertion sites of IVs, catheters, and medical devices, particularly when left in for a long period of time. Technically, these are not secondary infections, as they are due to the equipment rather than a primary illness. However, some may refer to them as secondary infections because they occur after the placement of the device.

Secondary Infection vs. Co-Infection

Secondary infections occur after, or because of, primary infections. However, sometimes people have multiple infections that aren't directly related to one another at the same time. 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. A person who is having condomless sex is more likely to be exposed to STIs. Which STIs, and thus the risk of co-infection, depends on which infections they are living with.

In contrast, if people are diagnosed 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. 

There is also a type of co-infection that is somewhat similar to secondary infection. Sometimes an STI such as herpes makes people more susceptible to HIV. In that case, the sores caused by herpes make it easier for HIV to get into the body. When a person acquires HIV in this circumstance, the lines become blurry. Most professionals consider this co-infection because the HIV infection isn't directly a result of the herpes infection.

You could make a case for calling HIV acquired in this way a secondary infection, but most doctors wouldn't. In part, this is because most secondary infections are treated alongside the primary infection. In contrast, HIV is treated as its own separate illness. It's also because in this circumstance, it's not clear that the person wouldn't have acquired HIV anyway. That's the major difference between a secondary infection and a co-infection. A secondary infection can't happen without the primary infection. With co-infection, it's possible that the primary infection just made things easier.