Sam came home from school with a cold caused by a virus and his sister, Ana, caught it from him. Sam got well and went back to school. A few days before Ana got well, Sam came down with a cold again. Their mother said that Ana had given the cold back to Sam. How likely is it that their mother is correct, and what does it mean for Ana's chances of getting sick again?

Question

anonymous · Answer

If Sam only had a mild infection of the virus he might not have acquired immunity to it and could catch it again, as could Ana if she had a mild infection.
		If Sam's body did not store up enough antibodies to the virus he could get sick with it again, as could Ana if the same thing happened to her.
		Sam could have picked up another virus at school as well as catching the same one from Ana again, so Ana could get sick again with either virus.
		Sam could not get sick with the same virus again since he has acquired immunity to it, so he must have a different virus which Ana could catch.