Four mutations in the gene encoding the protein that enables the coronavirus to penetrate animal cells were apparently the key difference between a virus that caused a SARS epidemic and one that caused a cold or no symptoms at all. Flu fighters suspect that a change of roughly the same size could make it far easier for flu to infect people. At present, it may be that 120 or so people infected with H5N1—most of them farmers or others in proximity to live fowl—got it by handling birds and then rubbing their eyes, which have higher numbers of the type of receptors that line the gastrointestinal tracts of birds, rendering them easily infected by the virus. Once in the body, flu can adapt to the receptors in our lungs, which is why it is crucial to immediately isolate anyone who has been infected with H5N1 and could theoretically be brewing a more deadly form of the virus.