Yes I do eat pretty healthily, but to give you an example, so did my friends who caught Measles in a mini-outbreak that flared up in my school when I was about 13. Afterwards it was shown that there was a pretty clear correlation between the kids that got sick and the parents who stopped their kids getting a Measles jab back in primary school.
There was one kid I knew who came from a pretty poor family and didn't really eat well. He was always a bit scrawny and unhealthy looking and yes, he did catch a lot of colds and things like that. But when a load of kids went down with Measles he was fine, because his parents let him have the free jab a few years earlier.
Eating well and looking after yourself can help with a lot of things, but if your body doesn't know how to handle something that you catch, you will get sick.