Influenza, often shortened to the flu, was a type of Earth virus. Some strains of the disease are known to be particularly deadly. As a very common disease, some references to the disease may reflect a confusion of the symptoms or a misuse of the term.
In 1918 through 1919, an outbreak of Spanish flu killed around five percent of the human population worldwide, more than World War I. (PROSE: The Clockwise Man, TV: Children of Earth: Day Four) It's possible the Dark Forces' dissipation was the cause of the outbreak. (PROSE: Casualties of War) Hugh Curbishley claimed that he had become restricted to a wheelchair as a result of the 1918 epidemic. (TV: The Unicorn and the Wasp)
In 1965, in exchange for twelve children, the 456 provided an antivirus for a brand new form of the Indonesian flu which would mutate into a deadlier form within four months and stood to wipe out 25 million of Earth's human population if unchecked. (TV: Children of Earth: Day Four)
In the 2000s, the alien virus carried by the toy Cuddlesomes was reported as a flu outbreak. Some believed it was specifically H591 Avian flu and Miranda Evenden suggested it came from toy factories in Asia. (AUDIO: Cuddlesome)
In late 2006 or early 2007, three months before the destruction of Deffry Vale High School, half of the staff became sick with the flu following the arrival of the new headmaster. (TV: School Reunion)
In December of 2007, Wilfred Mott was unable to attend his granddaughter's wedding to Lance Bennett because he was bedridden with a severe infection which his daughter referred to as "Spanish flu". (TV: The Sontaran Stratagem)