Scientists have warned that we have not seen the end of H1N1 swine flu that swept the globe in 2009/10. The strain of the virus could easily morph into a more transmissible form, while an older, mid-20th century virus could also come roaring back they say. Asian influenza, a H2N2 strain, first appeared in 1957 and killed one to four million people despite a major vaccination campaign. Studies have shown that most people today aged 50 or older retain some immunity to the virus, which continues to circulate in birds and swine. But the younger are more vulnerable they warn.