As for the question whether parasites is the correct word, a semantic analysis as yours does not consider the feelings of those who take the risks, albeit miniscule, and vaccinated their children. Thus, parasite sounds good, and freeloader or leech sounds even better.
You said: "Vaccination is the boy scout of medicine...". I just love this line. Always ready to help. Of course, sometimes the scout and the little old lady get mowed down by a speeding drunk, but, the scout was prepared.
I agree that my analysis might concentrate excessively on semantics over substance, but I think logically characterizing the lapses of non-vaccinators is important. I am wholly convinced that vaccination is one of the most important milestones in medical history, and a major cause for modern human flourishing. However, any time we force someone to do something against his will, there are both ethical and practical consequences. So we must distingish between people who benefit from other's actions from people who directly cause harm, from people who fail to prevent harm, etc.
In other words, I might prefer allowing people to benefit, via herd immunity, from my vaccinating my own children, despite the mild risks imposed on my family, over the inevitable law suits, growing conspiracy theories and other problems that stem from requiring the kooks to vaccinate their kids. For one example of the dangers of a backlash, read this note that I received from someone upset about some pro-vaccination comment I made on YouTube:
I'm assuming that you don't have a child who was injured by a vaccine then. Vaccines are toxins intentionally injected into babies and children FOR MONETARY GAIN OF PHARMACEUTICAL COMPANIES. The CDC knows exactly the harm caused by these vaccines. It's a scam - just like health insurance, cesarean sections caused by induction, and many other facts of American life. Vaccines aren't proven to work anyway. Vaccinate your own kids if you like, but mine are staying AWAY from those toxins!
If we force this person to vaccinate his kids, everything from the kid's future failures in school to his inability to make friends will be immediately blamed on vaccination (In this case, I'd blame the parent...). I'd rather the conspiracy theorist be forced to scramble to find something else to vilify.
Thank you, Probe, for reading my blog. For now, anyone who makes thoughtful comments will get personal responses.