Actually they start feeding immediately, but the bacteria which causes lyme disease lives in the ticks intestinal system (not all ticks carry the bacteria). After the tick is done feeding (in general approx. 1-2 days but can be longer or shorter), or overfeeds, it can ’vomit’ back into the host and possibly spread the bacteria from its intestines. This is also why you want to be careful when removing the tick and it’s best to use special plyers, and avoid squeezing the ticks body.