When I begin to discuss externalities in my Principles of Economics and my Environmental Economics classes, I usually use vaccinations as an example of a positive consumption externality. The economic issue of vaccination is not so much whether you are preventing illness in your own child by having him or her vaccinated, but rather are you taking into account the full social costs of not getting vaccinated when choosing whehter to vaccinate your own child. That is, by not getting vaccinated, not only do you take a chance of your own child contracting a disease, but you increase the probability that others will contract the disease as well. So by having your child vaccinated you provide external benefits that don't enter your own cost/benefit calculation. The result is social underprovision of vaccinations and socially inefficient levels of disease.
In the past decade, thanks to ill-informed celebrities (e.g. Jenny McCarthy) and pseduo-scientists (quacks) there has been a reduction in vaccination rates around the world. Now, the Council on Foreign Relations has provided a map to show the impacts of reduced vaccinations.