The Host

The Host derived from an alien entity that fell to a rural area of Scotland in 1540, where it landed in St. Catherine's Glen, near a monastery.

The Doctor described it as a "lupine wavelength haemovariform". surmised that only a single cell survived, which was then incubated in various hosts via biting them, passing down through the centuries until it matured. (DW: Tooth and Claw)

However, the Doctor's theory seemed contradicted by a scene of the werewolf attacking an innocent traveller in 1540. (Tardisode: Tooth and Claw)

The monks of the monastery, the Brethren, who turned away from worshipping God and began to worship the wolf instead, and kidnapped young boys to served as its host.

The werewolf was a large, hairy, wolf-headed humanoid. The infected Human would turn into a werewolf whenever it was exposed to certain wavelengths given off by the moon (though too much moonlight to overload its system). It was nearly twice the size of a Human and had increased strength, resilience and speed. Mistletoe would cause burning and twitches when the werewolf touched it, though this might have been a conditioned response deliberately created by the Brethren or a physical allergy. It's unknown what affect silver would have, since none was used on it. It was also colourblind, but had a stronger sense of smell to compensate.


 * The werewolf was also able to sense the Bad Wolf in Rose. It is unknown how he did this, but the message may have accidentally been sent down through future timestream.

Eventually, in 1879, the Brethren arranged for Queen Victoria to be diverted to the nearby Torchwood Estate where the current host would infect her with the wolf cell, leading to the creation of the "Empire of the Wolf". The plot was thwarted by the Doctor and Rose, who used a telescope constructed by the father of Sir Robert MacLeish against the werewolf. The telescope was in actuality a gigantic light amplification chamber, and using the Koh-i-Noor diamond as a focusing element, struck the werewolf with enough moonlight to dispel it from the planet. It is unclear, however, if Victoria was indeed infected in the end. (DW: Tooth and Claw)