Germs key part of Guns, Germs, and Steel

I may be mistaken but when Jerod Diamond wrote Guns, Germs, and Steel he did not study much about Africa. In the book Diamond takes an in depth look at the land, weather, and environment of each continent. He then draws conclusions about why some civilizations flourished and others were susceptible to attack. In the last volume of the video series Diamond does explore Africa in depth.

The questions he discovered were why were diseases, the germ portion of Guns, Germs, and Steel, affecting the colonizers more than the native inhabitants? The reverse had occurred in the Americas. The theory is that the way Sub-Saharan Africans (SSA) structured their towns prevented the easy spread of disease such as malaria and yellow fever. First of all they did not live close to the water like the Europeans did. Even today some women still carry water for miles in metal pots on their head. Secondly ethnic groups were relatively isolated. If someone get sick in one town it doesn't easily spread to another town. The fractal society is evident in the many different languages and the many different religions that are still alive today.

But modern day SSA is a reflection of European city structure. And disease is a major cause of the developmental problems of these countries. Disease keeps many children and adults in the hospital and not at school and work.

My questions are:
1. Can a mosquito not spread the diseases from village to village?
2. What about other tropical areas of the world? Did they not experience the same phenomenon?