A secondary infection is an infection which sets in during or immediately following treatment for another infection or disease. Secondary infections can vary in severity and frequency, depending on a number of factors including the health of the patient, the cause of the initial problem, the treatment approach used, and the conditions in the facilities where the patient is treated. Sometimes, secondary infections are common enough to be readily expected, while at other times, they can be unexpected and sometimes very frustrating for medical personnel.

One of the most common reasons for a secondary infection to occur is suppression of the immune system. For example, if someone takes an antibiotic to treat an infection, another infection might arise as bacteria take advantage of the situation, since the antibiotic kills beneficial bacteria which might stave off infection normally. A secondary infection can also occur as a result of some treatments. Prolonged placement of intravenous lines, for instance, can sometimes lead to infection.

A secondary infection is usually treated as a complication, and it is something which must be addressed, especially in a patient with a compromised immune system. HIV and AIDS patients, for instance, are very prone to secondary infections, and they can die as a result of infection because their immune systems are not able to function. Secondary infections can also be dangerous for cancer patients, people who have undergone extensive surgery, and people with other serious medical conditions.

Because secondary infections are a common risk, many medical providers have steps in place to identify the early signs of secondary infection, with the goal of addressing such infections quickly, before they have a chance to spread and cause problems for the patient. Fever, swelling, soreness, and discoloration of the skin are all treated very seriously in patients at risk for secondary infection, as these symptoms can indicate that an infection is moving in. In some cases, patients are even given prophylactic antibiotics to prevent the onset of secondary infection; this practice is very common before surgery.

In some cases, a secondary infection may be caused by unclean conditions or improper care. This is especially common in poor communities, which may lack the resources necessary to keep their clinics and hospitals as clean as they need to be, especially during disease epidemics. In a more wealthy community, secondary infections due to hospital error are often treated as grounds for a lawsuit, so medical personnel have another reason to prevent secondary infections in their patients.