According to a recent study of the 600 most commonly used mobile health apps for iOS and Android,

...only 183 (30.5%) had privacy policies.


Source: "Availability and quality of mobile health app privacy policies," Journal of the American Medical Informatics Association (JAMIA), August 21, 2014, http://jamia.bmj.com/content/early/2014/08/21/amiajnl-2013-002605.abstract?sid=a3656f74-25eb-469e-91a1-f6d5be627ca6