|Classification and external resources|
In parasitology, coinfection (//) is the simultaneous infection of a host by multiple pathogen species. In virology, coinfection includes simultaneous infection of a single cell by two or more virus particles. An example is the coinfection of liver cells with Hepatitis B virus and Hepatitis D virus, which can arise incrementally by initial infection followed by superinfection.
Global prevalence or incidence of coinfection among humans is unknown, but it is thought to be commonplace, sometimes more common than single infection. Coinfection with helminths affects around 800 million people worldwide.
Coinfection is of particular human health importance because pathogen species can interact within the host. The net effect of coinfection on human health is thought to be negative. Interactions can have either positive or negative effects on each of the coinfecting parasite species. Under positive parasite interactions, disease transmission and progression are enhanced and this is also known as syndemism. Negative parasite interactions include microbial interference when one bacterial species suppresses the virulence or colonisation of other bacteria, such as Pseudomonas aeruginosa suppressing pathogenic Staphylococcus aureus colony formation. However, the general patterns of ecological interactions between parasite species are unknown, even among common coinfections such as those between sexually transmitted infections.
A globally common coinfection involves tuberculosis and HIV. In some countries, up to 80% of tuberculosis patients are also HIV-positive. The potential for dynamics of these two infectious diseases to be linked has been known for decades. Other common examples of coinfections are AIDS, which involves coinfection of end-stage HIV with opportunistic parasites and polymicrobial infections like Lyme disease with other diseases.
See also 
- Infectious disease
- List of human diseases associated with infectious pathogens
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