The infection of a single cell by more than one virus.
Typically coinfection is seen, in the laboratory and with phages, as infection of a single bacterium by two or more phages of the same phage type. It is possible, however, for multiple phages to infect a single cell where those phages are not of the same type and in fact it is fairly typically for phages to infect bacteria that are already infected lysogenically, with coinfection therefore occurring between a newly infecting phage and a preexisting prophage.
See superinfection, a term that is similar but not identical to coinfection. See also secondary phage, secondary adsorption, or secondary infection.
Note that coinfection does not necessarily follow co- or multiple adsorptions of a single bacterium since phages can display superinfection exclusion, which means that later phages are blocked from entering the bacterium's cytoplasm. (I use here the phage genome reaching the adsorbed bacterium's cytoplasm as a working definition of "infected".)
Note also that under most circumstances the burst size of a coinfected bacterium is either the same as that of a singly infected bacterium or declines relative to a singly infected bacterium. The latter can be described as a depressor effect.
Coinfection also can be defined more generally in terms of infection of especially multicellular organisms by more than one type of pathogen.
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