Well in order to avoid infectious diseases...
The best answer would be
A.) practicing good hygiene.
The reason why this is the best answer, is because with our hands we touch everything. And that way we can bring infectious diseases to our mouth and eyes or any where the disease has a place to enter.
The reason why B, C and D aren't the answers are because...
B.) avoiding contact with vectors can be really hard when you can't see the vectors.
C.) boosting your immune system with immunizations, if too many immunizations are given, then you could get sick, with high and dangerous temperatures.
D.) living in a heavily wooded area... I dont think this is good because it could be expensive so suddenly changing the place where you live, to go to the forest or something.
Anyways good luck! :)