Re-infection is a term used to describe a new or secondary infection by a virus that has already infected a person. In most viral diseases, re-infection with the same virus doesn\'t occur because once the immune system conquers the original viral infection, it creates immunity against that virus. Re-infection occurs almost constantly, however, in some types of infection, such as the cold or flu viruses, because each new version of those new viruses is substantially different from the last. This is why a person may develop immunity to the flu strain that is common in one year, but still be at risk from the strain that becomes dominant the next year.