Coinfection

Coinfection is the simultaneous infection of a host by two or more pathogen species or strains. It is distinguished from superinfection, in which a new pathogen infects a person already infected or colonized by another pathogen, often after antimicrobial therapy. Coinfections can involve viruses, bacteria, fungi, or parasites and may occur in humans, animals, and plants.

Within a host, pathogens may interact cooperatively or competitively, affecting replication, immune response, and disease severity. Interactions can be synergistic, leading to more severe illness, or antagonistic, limiting each other’s growth. The clinical presentation may be dominated by one infection, while the other remains undetected, complicating diagnosis and management. Coinfections can alter diagnostic test performance and may require multiple laboratory assays to identify all pathogens involved.

Epidemiology and risk factors: coinfections are more common in settings with widespread exposure to multiple pathogens, in individuals with immunosuppression (such as HIV infection, cancer, or organ transplantation), malnutrition, or advanced age, and among hospitalized patients or residents of endemic areas.

Examples and implications: common coinfections include HIV with hepatitis B or C virus, tuberculosis with HIV, and respiratory viruses with secondary bacterial pneumonia. Coinfections can influence disease outcomes, transmission dynamics, and response to treatment, and they pose challenges for clinical management, including drug interactions and competing therapeutic priorities.

Diagnosis and management: diagnosing coinfections relies on targeted testing for multiple pathogens when clinically indicated, using molecular assays, serology, antigen tests, and cultures. Management requires careful selection of therapies to address all pathogens, monitor for adverse effects, and consider interactions. Prevention relies on vaccination, infection control, and public health surveillance to reduce exposure and transmission.