Scholars have suggested, among other reasons, disease, political upheaval, overpopulation or drought.

UNESCO: Culture

The curious life cycle of the parasite, called plasmodium, is one of the reasons why the disease has proved so difficult to control.

BBC: Venom to arm mosquito against malaria

Cancer is a very lonely disease and one of the reasons why it feels lonely, seems lonely, is this axiomatic belief that it's not transmissible.

NPR: Startling New Evidence Cancer Is Contagious

The goal of the U.S. vaccination program should be to eliminate meningococcal disease, and the preferred strategy to eliminate the disease would include infant immunization, even though for reasons that are unknown the incidence of the disease has declined in recent years.

FORBES: Immunizing Bureaucrats Against Complacency

Komen for the Cure foundation, investigated eight locations in an effort to identify common reasons for their above-average mortality rates from the disease.

NPR: Closing the Gaps on Breast Cancer Mortality Rates

That's because obesity leads to so many different diseases--diabetes, heart disease, arthritis, mental disease--that it's hard to separate out the reasons for the reduction in costs once the employee loses weight.

FORBES: Watching The Corporate Waistline