Dalek factor

The Dalek Factor was the distillation of the essence of what distinguished Daleks from other species (primarily humans).

Development
The Daleks recruited the Second Doctor, against his will, to isolate and identify the Human Factor, using Jamie McCrimmon as a test subject. The Daleks claimed that they wished to distill the most important qualities from the Factor and use it to make themselves more deadly. The Daleks, in fact, planned all along to use the Doctor's work to identify the Dalek Factor. (TV: The Evil of the Daleks)

The Dalek Plan
The Factor took the form of the "virtues" of total, instinctive obedience and rabid xenophobia. Meanwhile, the Daleks had managed to experimentally transfer the Human Factor to test Daleks, creating three humanised Daleks.

The Daleks planned to use the Second Doctor and his TARDIS as a means of spreading the Dalek Factor throughout history to prevent the Great War which began after their failure to destroy the solar system with the Time Destructor and threatened the total defeat of the Dalek race. (PROSE: The Evil of the Daleks) The Factor was to be weaponised in the form of a vapour which would be spread throughout Earth's atmosphere. They initially used a machine in the shape of a metal archway as a means of instilling the Dalek Factor; anyone passing through the archway acquired it. The Daleks succeeded in instilling the Factor into Theodore Maxtible. When the Doctor passed through the archway, however, his non-human nature made him immune and he sabotaged the machine so that it would convert any Dalek who passed through it into a humanised Dalek. A civil war then broke out between the humanised Daleks and the Daleks loyal to the Emperor. (TV: The Evil of the Daleks)

Toward the end of the Last Great Time War the Daleks sent a time capsule to 70 AD Earth in order to spread the Dalek Factor amongst the humans. In this instance the Factor took the form of a distinctive set of genetic changes. The capsule's engine exploded, and the Dalek was killed soon after impact. It was only able to release a small amount of the Dalek Factor, leading to only 1 in 500,000,000 humans having it. The only known carrier of the Dalek Factor was Kate Yates.

The Dalek Factor was generally hidden amongst the genes. When the casing of the Dalek was rediscovered and activated in 2007, the Dalek Factor in Kate was reactivated. When Kate Yates was hit by a car, she was able to survive and regenerated the damage away. Her hair turned blond and she gained immense intelligence and increased strength. She also started to develop a Dalek mindset, hating all other humans and trying to help the Daleks. When she touched the casing of the Dalek, Kate was also able to reactivate it and cause a new Dalek to be grown from raw materials in the case. Eventually, she overcame her Dalek instincts and was able to help the Tenth Doctor and Rose kill the Dalek, causing her Dalek Factor to deactivate. (PROSE: I am a Dalek)