translation.en: 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 (so called recombination) and acquired the ability to both infect humans and, more importantly, spread between them.
translation.sv: Minst några av de tre pandemierna under det senaste århundradet (1900-talet) tros ha härstammat från ett djur- eller fågelinfluensavirus som muterades eller utbytte gener med en mänsklig influensastam (så kallad rekombination) och därmed både kunde smitta människor och, ännu viktigare, spridas mellan dem.