Geography can be defined as the study of how various things are situated in space across the face of the Earth. This means that geography can be used to study diseases like AIDS and cancer by looking at how the diseases are distributed across the Earth and at how factors that could affect the diseases are distributed across the Earth.
The first thing to do when studying these diseases with geography is to look at which areas of the world have greater or lesser concentrations of the diseases. Typically, you might map this on a map of the world or of a given region that you wish to study.
Next, you would want to identify factors that you think might affect the likelihood of people having these diseases in a certain area. For example, if you were studying cancer, you might want to think about what sorts of pollutants are found in various areas. You would then want to map out the ways in which these factors are spread across the world.
Then, you would see if there are correlations between the factors you have identified and the prevalence of the disease. Are the areas that have the most pollutants also the areas that have the highest rate of cancer? In this way, you would be using geography to study the disease and its possible causes.