Coinfection refers to the simultaneous infection of a host by two or more different pathogens or strains. The interacting organisms may be viruses, bacteria, fungi, parasites or combinations thereof, and their coexistence can influence disease outcomes.
Explanation
When multiple pathogens occupy the same host, they can interact in ways that alter pathogenesis, transmission and immune responses. Coinfecting organisms may compete for resources or niches, synergize to enhance replication or suppress host defences, or modulate each other’s virulence. For example, influenza virus damages respiratory epithelium and predisposes individuals to secondary bacterial pneumonia by Streptococcus pneumoniae or Staphylococcus aureus. HIV‑infected individuals are more susceptible to tuberculosis, hepatitis viruses and opportunistic parasites due to immunosuppression. In malaria‑endemic regions, concurrent helminth infections can skew immune responses and affect malaria severity. Coinfections also complicate diagnosis and treatment because symptoms may overlap, and therapies directed at one pathogen can impact the other. Understanding coinfection dynamics is important for public health, as control measures targeting a single pathogen may not be sufficient in settings where multiple infections are common.
Illustrative cases
Respiratory viruses such as SARS‑CoV‑2, influenza and respiratory syncytial virus can coinfect patients, potentially worsening disease severity. HIV and hepatitis C virus coinfection accelerates liver disease progression compared with either infection alone. Individuals with human papillomavirus and herpes simplex virus may have higher risk of cervical lesions. In veterinary medicine, pigs coinfected with porcine circovirus and porcine parvovirus exhibit more severe reproductive failure than those with single infections. In plant pathology, mixed infections of viruses can increase plant damage or facilitate virus transmission by vectors.
Recognizing coinfections emphasizes the complexity of host‑pathogen interactions and the need for integrated approaches to diagnosis, therapy and prevention.
Related Terms: Superinfection, Secondary infection, Comorbidity, Microbiome, Pathogen