You may begin with:

• Statista - quick statistics and articles related to the topic
• Incidence & Prevalence Database - includes world's epidemiology data 
• National Center for Health Statistics - US health related data portal
• Healthy People.gov - rich data on various US health related topics
• Eurostat Health data - Tip: may also go to the statistics a-z index for more health related databases
• World Health Organization (WHO) Health Statistics and Information Systems 

 Additional resources:

• Specific disease related organizations often provides disease specific data. To find those organizations, use Encyclopedia of Associations
• You may also check individual countries' related government sites for health statistics