Young children are the worst for intestinal worms, head lice, whatever. This is because they touch other children a lot and more likely to put hands in mouth and so on. Doesn't mean they are dirty just easier to transmit stuff.

I used to go round to my friend's place a lot and he had children so I ended up with threadworms. I asked my doctor who sent me to the pharmacy for a simple remedy, some powder which you dissolve in a glass of water and drink. Instantly kills the worms in your gut but their eggs will not die. You have to wait two weeks for the remaining eggs to hatch then drink the stuff again and you're all set.

Might be too much information but if you're wondering how do you know if you have threadworms it is because your arse will itch like crazy especially at night. That's how they transmit to other hosts - scratching yourself because of the itch will get eggs on your fingers and lack of hygiene does the rest.