Libertarians, what is the difference between military defense and health defense?

I have a question for the libertarians out there. It is the libertarian position that government should be restricted to ensuring an orderly place to conduct business and nothing more. That is, defense of its citizens only letting the free market handle everything else, whatever that's supposed to mean. As I understand it, this means the government has control over things such as public safety, military, and nothing more. Libertarians often say that healthcare, for one example, is off limits and that it should be left to charity and the private sector.

So if the government's duty is merely defense of its citizens, why wouldn't this include defense from disease as well? What is the difference between defense from militants and criminals and defense from disease? Both are a threat to the order within the marketplace, the former can be considered disease in a metaphorcial sense, and disease actually kills a heck of lot more than any war. Heart disease is a bigger threat to the USA than terrorists ever were.

You might say "It's not mine or anyone else's obligation to pay to ensure your health. You pay for that yourself, everyone for themselves, free enterprise, etcetera."

Why don't you say "It's not mine or anyone else's obligation to pay to ensure your defense from killers and military threats. You pay for your own mercenaries to defend yourself, everyone for themselves, free enterprise, etcetera."

As I see it now, I am not impressed with the libertarian ideology and this is one of several problems I have spotted in it. Libertarianism seems to cherry pick without any specific reasoning the roles that government should have.

Perhaps I am missing something. If any libertarian out there has something to add, by all means.