It’s a dual problem that creates a big one.
Unimmunized hosts are like dry leaves in a forest. The green (vaccinated folk) won’t burn and the dry won’t burst into flames without cause. The illegal is just the discarded cigarette butt, unwatched campfire, etc etc. Once the dry leaves get burning the green follows.
The solution is to vaccinate your children.
The Measles Vaccination Rate in The United States is 91%
The Measles Vaccination Rate in Mexico is 89%
Now, pease explain the mathematics behind illegal aliens being responsible for this Measles outbreak.