Finding epidemiological information can sometimes feel hit or miss for your disease, population, or study design. 

For population: Consider what mesh terms you are using in Pubmed. Consider the ways people have been historically misrepresented or oppressed using language, as unfortunately this can exist in the medical literature as well. See "Too many scientists still say Caucasian."

For disease: Have you conducted a google search to see if there's a national, international, or global organization that collects data on your topic? Consider a simple search like "asthma organizations," and use your discretion to evaluate the websource. Have you done a search in a specialty epidemiology or disease-related journal? The Journal of Pancreatic Cancer is an example of such a journal.

For study design: Have you used the publication limiters within Pubmed? Have you tried including "cohort study" or "observational study" or the particular type of study design you want to retrieve as a keyword in your search string? Do you know how to visually identify an epidemiology study/paper?