How Can a Libertarian Be In Favor of Mandatory Vaccination, But Not Mandatory Health Insurance?

Just a quick note, because this has popped up in the comments.  I am not against public health efforts when the behavior of one person puts another at direct physical risk.  You cannot drain your toilet directly into the local water table even if it all happens on your property, and you do not have a right to expose others to tuberculosis.  Similarly with vaccines.  The government does not have a right to mandate vaccination for your own good.  But it does have a right to do so when being unvaccinated is a physical threat to others who engage in normal behavior.

In our society, we have defined "having sex with someone who has ever had sex with another person" as a normal behavior--yes, even devout Catholics believe it is fine to marry a late convert, or a widower.  When 50% of the population has had the disease in question, and it is transmitted by activities that we consider a normal part of life, it's within the realm of public health.