Borrelia burgdorferi persists in the tick's gut between blood meals. When feeding begins, the bacteria respond to the influx of blood by changing surface proteins, multiplying and moving through the gut wall to the salivary glands. Only then can they enter the host with the tick's saliva.
Animal studies and public health guidance converge on the same practical consequence: transmission is uncommon in the first day of attachment and rises substantially thereafter, with 36 to 48 hours commonly cited as the interval after which risk becomes significant. Prompt removal of an attached tick therefore prevents most Lyme infections, and health agencies recommend daily tick checks in endemic areas for exactly this reason.
The delay is not a guarantee, and it does not generalise. Some other tick-borne pathogens, notably Powassan virus, can be transmitted within hours because they are already present in the salivary glands. The precise minimum attachment time for Borrelia transmission in humans has not been established, and the widely quoted figures come mainly from animal experiments.