Non vaccinated children a risk to vaccinated children...explain this to me

Ok so this NOT meant to be a debate on the whole vaccination issue. I am sure every parent has their child's best interest at heart when they make these decisions.

I was reading a news article about an increase of disease risks because of some parents choosing to not vaccinate their children. Ie. a return of old diseases. I was reading the comments underneath and quite a few parents were claiming that these non vaccinated children are a risk to all the other children etc etc etc.

What I don't understand is that I vaccinate my child so that she isn't at risk of these diseases :doh:
How does a non vaccinated child now increase her chance of getting say measles if she's vaccinated?

Please explain...I am baffled....

A vaccination decreases the likelihood of your child getting a disease but it is not 100% effective. If your child is exposed to a contagious disease, even though they are vaccinated there is still a risk they will get that disease.

Non vaccinated children increases the prevalence of the disease because they have less immunity to that disease than a vaccinated child. Vaccination is not 100% effective, so when the disease is more common then the amount of vaccinated children that also get the disease will increase. However, the effects are apparently less in a vaccinated child.

I was vaccinated and had both measles and mumps as a child.

ETA, they may also worry about younger children that have yet to be vaccinated, such as whooping cough in a newborn.

Ok I see I see. At least I can understand their viewpoint now, couldn't work it out :)