Dual infection is when a person is infected with two or more strains of HIV. That person may have acquired both strains simultaneously from a dually infected partner or from multiple partners. A different strain of the virus is one that can be genetically distinguished from the first in a “family” or phylogenetic tree. Acquisition of different HIV strains from multiple partners is often called co-infection if all the virus strains were acquired prior to seroconversion, that is, very early before any HIV infection is recognized. Acquisition of different HIV strains from multiple partners is called superinfection if the second virus is acquired after seroconversion when the first virus strain already has been established. Superinfection and re-infection mean the same thing. Dual infections can be sequentially expressed, which can make co-infection look like superinfection. Sequentially Expressed Dual Infections (SEDI) may occur because immune responses against the predominant virus may allow other virus strains in the body to be expressed. Random shifts in evolving virus populations can also occur, which could look like superinfection even though dual infection was present from the beginning.