Refusing vaccines has become a very contentious point, to put it mildly. While most people are vaccinated against diseases like measles, some people can't be vaccinated (due to medical problems like poor immune function or due to age, like infants), and even those who are fully vaccinated have a tiny, tiny chance of the vaccine not working as well as it should. So when someone purposefully does not vaccinate their child for personal reasons, rather than actual medical problems, that increases the risk of those who can't be vaccinated becoming ill.
For parents whose children can't have the vaccine, this is a frightening prospect. And because the spread of the disease -- if those can't-be-vaccinated kids get the disease -- was due to someone purposefully avoiding vaccination and then letting their ill child loose in the world, does that mean you could sue that parent and other adults involved for medical malpractice? It's a tricky line to walk because you might not be able to sue the parents for medical malpractice, but the doctor who wrote the exemption for school may be another matter.
Malpractice vs. Reckless Endangerment
A charge of actual medical malpractice is usually reserved for those actually claiming to practice medicine. In this case, the parents technically wouldn't actually be trying to give medical advice that went sour. They may have made very bad decisions, but what they did would not likely qualify as actual malpractice. (Reckless endangerment or another charge might be possible.)
However, not vaccinating against something highly contagious like measles and then letting a possibly ill child run among crowds containing those who can't be vaccinated could be seen as a lack of reasonable care. After all, the parents are responsible for managing the medical decisions for their child, and if they take the attitude that their medical opinions will have certain effects that end up hurting you, that's a gray area. You may even want to talk to a medical malpractice attorney about whether this may count as malpractice.
The Physician Who Wrote the Exemption
Now, someone who could very well be at risk of a medical malpractice lawsuit from you is the doctor who wrote the exemption. Chances are the unvaccinated child needed an exemption for their school.
It is possible that the doctor wrote the exemption in good faith. But state officials are now finding that these medical and personal exemptions are often written for many children by only a few doctors in the area. For example, in San Diego, officials reported that one doctor wrote about one-third of all the medical exemptions for children in that city's school district after California started eliminating personal-belief exemptions.
If you find out who wrote the exemption, medical or otherwise, speak with a malpractice attorney. The attorney will have to investigate whether the doctor truly thought the child had a medical exemption (and patient privacy laws may interfere here), but if that child starts an outbreak, officials will likely trace the trail of exemptions involved. And you could end up with a possible malpractice target whose negligence or lack of reasonable care contributed to your child becoming ill.
Again, this is all very up in the air. But a malpractice attorney can help you sort it out.