This is one of most appalling of all anti-vax arguments.
Contracting and overcoming [childhood] diseases are part of a developmental process that actually helps develop a healthy, robust, adult immune system able to meet the challenges that inevitable encounters with viruses and bacteria will present later on.
Shawn Seigel at https://www.facebook.com/vaccineinfo has a lot to say on the subject. One example:
The irony of vaccination is that the industry claims it protects against disease, but disease is a natural, protective function; disease is detox.
This is an argument that is so ridiculous that you’d think it would simply fade away. The idea that it’s a good thing for you child to suffer and risk death or disfigurement is about as low as the anti-vaxxers can go, but their blind believe in the vast global vaccine conspiracy is so all pervasive that sickness and death seem to them to be preferable to vaccination.