They're picked up in areas where sheep and deer graze, generally in spring through to autumn. In my experience, they tend (but not always) to go for short-haired areas such as the head but I've found them on Amber's side before now. I would check a dog over thoroughly after walking in tick-like places. Left unnoticed, they can be fatal. Warning signs are things like fever, lameness, loss of appetite, lethargy, depression, cough. I think this occurs in extreme cases, I've only ever known of one dog getting sick and he was a big, long-haired GSD, so the ticks weren't easy to spot!
You can find out more at this site:
http://uk.merial.com/pet_owners/dogs/dog_flea.html but it is an advert for Frontline!