How did the 1918 flu virus infect humans? Well, viruses use “spike-like” molecules to attach themselves to cells, and apparently there were very few differences between the molecules in bird cells and those in humans for that particular virus, so it jumped pretty easily. This article from Time Asia explains what’s going on with avian flu in scientific and economic terms. The current US edition has some very good graphics depicting the molecules, but unfortunately those graphics are not on line, which is annoying.