In the flu pandemics of 1918, 1957 and 1968, the pandemic virus completely replaced the circulating seasonal flu.
In previous pandemics, the virus has then mutated, making its effects worse. So far H1N1 has acquired no obvious new mutations, but a few ominous signs have emerged.
A mutation to the virus's polymerase enzyme, which makes it replicate more efficiently, has cropped up in a sample from Shanghai, China. This could spread if it makes the virus more contagious. But it may also increase pathogenicity,
two cases of swine H1N1 with resistance to the main antiviral drug, Tamiflu, were discovered in people using the drug. Another was discovered in a girl who had never taken the drug, suggesting Tamiflu-resistant swine H1N1 might already be circulating.