What is sick made out of?

This would depend on your definition of 'deadly'.

"Any hemorrahgic fever, like Hanta,

Dengue or Ebola. Viral diseases that can kill a weak person in hours after the first symptoms appear."

May be considered 'most deadly' as they kill quickly and horribly, as well as being highly contagious and hence have the ability to kill all humans within the virus's range.

Historically, diseases due to lack of hygiene, such as the Black Plague (spread through lice on rats inhabiting human residences), cholera and typhoid, whooping cough, Tuberculosis and other bacterial diseases have been very deadly.

In the modern world Parasitically carried disease, especially malaria, and virus's such as HIV, are probably responsible for the most deaths, even where medical assistance is available.