1B is enough to take over Equatorial Guinea... the failed 2004 coup attempt was financed with a pathetic 20M. This says that the military budget of EG is 150M/year (http://www.globalsecurity.org/military/world/spending.htm) and I suspect that is a very high estimate.
It would be easy to invest in modern pharmaceutical manufacturing plants in African nations with generous IP laws and then start making modern HIV medications available for practically nothing.
Curing cancer is more of a crapshoot, but throwing several hundred billion at the problem would be a good start, if nothing else.
Proper distribution of the money remaining could go a long way towards eliminating extreme poverty.
