At least some of the three pandemics during the last (20th) century are thought to have come from an animal or bird influenza virus that itself mutated or swapped genes with a human influenza strain (so called recombination) and acquired the ability to both infect humans and, more importantly, spread between them.
Prevladuje mnenje, da je najmanj dve izmed treh pandemij v zadnjem (20.) stoletju povzročil virus živalske ali ptičje gripe, ki je mutiral ali zamenjal gene s sevom humane gripe (tako imenovana rekombinacija), s čimer je pridobil sposobnost, da okuži tudi ljudi in, kar je pomembneje, da se širi med njimi.