Knowingly harboring and transporting forms of life that are by nature dangerous maybe?
<<<Not in favor of forced vaccination, just thinking about a rationale.
I mean, I guess you could argue that you are responsible for the actions of your dog if it bites someone. Maybe you could argue that you are in possession of dangerous animals if you knowingly refuse to get a vaccination and someone gets sick as a result.
Remember how AIDS got its foothold in the U.S. originally? A large portion of the first cases were most likely because of this asshole -
http://en.wikipedia.org/wiki/Ga%C3%ABtan_DugasDude was a sociopath who knew he was sick and didn't give a shit. Had sex with hundreds of people.
Obviously there's no vaccine for AIDS and people didn't even know what the fuck at the time, but you get where I'm going with this.
If you are walking around with a big open bucket of poison and it's slopping all over the place and splashing people, you have culpability.
Can I force you to seal the bucket or stop carrying it? Possibly in self defense, yeah. Arguable, obviously.
I am glad we have cures and vaccinations. Whether you take them or not is... Well, up to you. Having said that, if your kid shows up to school (Public or private, doesn't matter.) with fuckin' polio or smallpox or some shit because you didn't get the little yuppie larva a vaccination, you'd be responsible for all of those scarred up and crippled kids, if you ask me.
*Shrug*
I imagine in a voluntary society you probably couldn't get around very well in cities, what with the "No vaccinations, no service." signs everywhere.