Secondary Infection vs. Co-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. 

Types of Secondary Infection

There are several ways that a primary infection can increase susceptibility to disease:

  • Some diseases alter the effectiveness of the immune system. This can make it easier for a 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. 
  • Scratching the sores caused by skin STDs such as molloscum contagiosum can also cause secondary infections. 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. (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 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. This is because they are secondary to the placement of the device.

    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. A person who is having unprotected sex is more likely to be exposed to STDs. Which STDs, and thus the risk of co-infection, depends on which they're infected with.

    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. 

    There is also a type of co-infection that is somewhat similar to secondary infection. Sometimes an STD 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 gets 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 gotten 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.

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