How would life expectancy impact the calculation of disease prevalence? I want to understand how to make calculations on the prevalence of a disease in a country population and the impact that the element of average life expectancy (of those suffering with the disease at time of diagnosis) has on this calculation. 
Are there any 'best practice' papers on making epidemiology calculations? 
 A: It depends on the disease. Consider the following formula:
Prevalence = Incidence * Duration.
If the disease state is permanent, like HIV, then a decrease in life expectancy will decrease prevalence. Example:
We have a population of 1000 people. 5 people get the disease, so incidence = 0.005. If the disease is harmless, and everyone lives, then there are 5 prevalent cases, so prevalence is also 0.005.
What if the life expectancy in the country drops dramatically - specifically, to a very, very small number. Now the duration of the disease is reduced, because people die. Now, even though incidence is 0.005, since duration ~ 0, prevalence will also ~ 0.
This is a problem with the HIV statistics in some African countries. As we introduct HAART, life expectancy is going up, which means the duration of the disease is going up, so you see a rise in prevalence even if your number of incident cases is staying steady or dropping.
For a "best practices" book, I'd suggest "Epidemiology: An Introduction" by Ken Rothman to start with, and "Modern Epidemiology 3rd Edition" by Rothman, Greenland and Lash once you feel like moving on to more advanced topics.
