saliva
I *believe* what this is, is actually just because of the diet of garter snakes. In, um, texas and florida (I think) there are these smallish toads with mildly toxic secretions. Water and garter snakes aren't much bothered by this, and eat the toads anyway. Some people, however, can have rather nasty reactions to this toad slime. So if a garter snake that's been eating toads bites a human, there's a fair chance of getting some redness and swelling.
In a very, very few cases, such a bite on a very toad-allergic person can look kind of bad at first, enough to be confused with the swelling from an early-stage venemous snake bite. So there have been a few cases (according to one half-remembered discovery channel show) where people with bad swelling get rushed to the hospital for antivenin, and it turns out they've really been bitten by a garter.
Anyway, if you were to feed a wild-caught garter anything but toads for a while, you'd probably be safe. Captive born garters would also be safe.
Anyone know what the whole story is?