Can I get a Master's in Economics with a Bachelor's in Mathematics? I am going to receive a Bachelor's of Science in Applied Mathematics soon. I found out facilitating medical environments is what I found interesting after volunteering at a hospital for two years and getting to meet an administrator and a medical actuary. 

I have ZERO background in Economics, minus basic Macro and Microeconomics AP credit that I still remember. I can program really well, and I'm a very, very strong mathematician. I have 4 total undergraduate publications in Numerical Solutions to PDEs (1), Set theory (1), and Mathematical Models in Ecology (2) using differential equations. Although none are economics per se, friends who I have talked to say that economics is highly mathematical and involves rigorous programming, so these tools will serve me well. 

I have a 3.98 GPA in Mathematics, have relevant tutoring experience on a campus math help center, etc. Is a graduate degree in healthcare economics or even an MPH with a focus in healthcare economics viable with an undergradaute degree in mathematics?
Mathematics is a good undergraduate major for a prospective economics grad student. And I suspect that you could gain admission to (and succeed in) a master's program somewhere with the background you already have. But graduate programs in economics do generally expect that you've taken at least some upper-division economics courses, so you would be a much stronger candidate for a master's program (or even a PhD program) with a bit more preparation.

For example, here's what UConn (where I earned my MA) has to say about application to graduate programs in economics http://econ.uconn.edu/grad/apply/faqs/:


  Do I need to be an economics major to apply?
  No, though some economics background is preferred. Students with little or no economics training, however, will usually only be considered for admission to the master’s program and, if admitted, will generally be required to take some undergraduate economics courses before beginning graduate study.


So, I think the answer to your question is "yes" for at least some values of "master's in economics". Completing intermediate level microeconomics and macroeconomics (or, if you can only take one, micro) would probably improve your chances of acceptance, though, not to mention giving you a better idea whether this is something you want to spend a year or two on.

From your description of your interests, though, it sounds to me as though a MPH is a better fit for you.