four - contagious
Martha scanned the alien laying in front of her on a makeshift hospital bed. She'd normally be unfazed by his purple pallor if it weren't for the green complexions of all the doctors and nurses stumbling around blindly.
"I don't understand," the alien doctor kept mumbling. "It just doesn't make any sense." Martha peered behind his neck where black ooze dripped onto the blue cloth beneath him. "This illness - this disease - it's impossible." Martha turned to the bewildered doctor, stern and serious.
"A good doctor doesn't rattle on about how a disease is impossible. They treat it."
"And you'd know?"
"I would, actually. Doctor Martha Jones." She paused, looking at the unconscious alien. "If I treat him, will you let my friend go?" She asked.
"You can't treat him. Anyone who has touched a patient has contracted the illness. There's nothing to do except let the illness die with the patients."
"I'm immune, trust me." She tentatively touched the alien's neck, taking a pulse. Steady. Strong. There was still time. "I'll help you, but you have to let the Doctor go? I mean - my friend. The one you've kidnapped." The alien doctor paused for a long time, considering the offer.
Without looking away from Martha, he addressed the nurse, "go get him. And be quick about it."
Quickly, Martha set to work.