Even if the established/widely accepted standards of ethics made human testing in lieu of any animal testing the clear ethical choice, it still would not be practical.
Think about it, how would you work on potential cures of ebola in the (several times many year long: http://en.wikipedia.org/wiki/List_of_Ebola_outbreaks) gaps between ebola outbreaks? Deliberately infect humans to be used as test subjects? Put all research on hold until the next outbreak came around?
You'd be able to find plenty of human test subjects for diseases like malaria or HIV, but for many diseases you would rapidly exhaust your supply of sick humans.
