Back in 2003, the National Assessment of Adult Literacy (NAAL) measured health literacy of adults in America. This was the first time American adults were tested on their ability to read, understand, and apply health-related information in English.
The performance levels were: Below Basic, Basic, Intermediate, and Proficient. More than 50 percent had “intermediate” health literacy. About 22 percent scored “basic,” and about 14 percent had either “below basic” or “proficient” health literacy. Women in general scored higher than men. Adults age 65 and above had lower average health literacy than younger adults.
Here’s the report: http://nces.ed.gov/pubsearch/pubsinfo.asp?pubid=2006483