MedlinePlus

https://www.nlm.nih.gov/medlineplus/
MedlinePlus is the National Institutes of Health's Web site for patients and their families and friends, produced by the National Library of Medicine.

Here is a link to a blog post describing some MedlinePlus search tips:
http://blogs.scientificamerican.com/observations/2014/03/31/the-quest-how-to-medical-librarian-search-for-free/