You can'd donate blood after malaria prophylaxis (it was three years around
here, back in 1990) because the chloroquine and other malaria-suppressant
drugs do not prevent you from getting malaria, they just keep the parasite
from reproducing so much that you have symptoms.  So you still *get*
malaria, and even though most people take additional, different drugs when
they get home that are supposed to kill the life-cycle of the parasite that
can encyst in your liver (and hide for years) -- they still want to be sure
that you aren't going to pass malaria on to someone via a blood donation.
So they figure if you haven't gotten sick again for three years, then you
probably don't have any parasites in your blood OR encysted in your liver.

Kathy Dettwyler