The idea, at the moment, is that the infections aspects are only present directly from the tap, so to speak, and that the magic dissipates after being exposed to air for a short time.  It would take a significant amount to fully turn a drinker into a cowpony, and an amount less than full transition would cause temporary changes.

Because the magic would only be present enough to cause changes while still in Rarity, a drinker would have to drink straight from her teats unless the milk was bottled in a near vacuum such as what could be done with a milking machine….