Bacteria (Doctor Who and the Silurians)

A colony of bacteria was kept and used by Silurians when apes raided their crops, and later in an attempt to exterminate humans upon waking from hibernation. (TV: Doctor Who and the Silurians) It caused what was later known as the Silurian plague. (AUDIO: Hosts of the Wirrn)

In the 20th century, early on in the Third Doctor's time at UNIT, a group of Silurians attempted to use this bacteria to wipe out most humans on Earth, so that they could repopulate it themselves. The bacteria were very fast acting, killing those infected within hours. One infected person, Edward Masters, left Wenley Moor, spreading the disease to London. Later, Brigadier Alistair Gordon Lethbridge-Stewart received word of the first case in Paris. At Liz Shaw's suggestion, the Doctor tried using the drugs denoted A37 and Z19 against the bacteria and found it to be effective. (TV: Doctor Who and the Silurians)

According to another account, the plague was caused by a virus rather than bacteria. (PROSE: The Scales of Injustice)