Indonesian influenza was a type of the influenza virus.
In 1965, the British government was contacted by the 456 who claimed that a brand new strain of Indonesian influenza would mutate within four months and kill twenty-five million people. Research backed this up with the virus being compared to the 1918 Spanish flu outbreak. In exchange for twelve children, the 456 offered an anti-virus. After the children were turned over, the 456 transmitted a successful anti-virus to the human race.
In September 2009, the 456 returned in search of more children. When they did, Jack Harkness related the story of first contact with the 456, noting that the anti-virus worked and the 456 appeared to keep their word. (TV: Children of Earth: Day Four)