Yeah, the aliens came down, abducted us, probed us all with with giant alien probes (anyone ever see the South Park episode where Cartman was abducted by aliens?)

and then they infected us with the alien microbes to get us ready for the colonization of the planet...

oh wait...that's the plot for an X-Files episode. (loved that show)

But, blaming aliens is a good excuse, just as good as blaming the Epstein Barr Virus, Lyme and genetics.