Final answer:
The painful, localized bacterial infection of the hair follicle is known as a boil or furuncle, requiring good hygiene and sometimes medical treatment.
Step-by-step explanation:
An acute, localized bacterial infection of the hair follicle that produces constant pain is a boil, also known as a furuncle. This type of infection presents as a painful, pus-filled bump on the skin caused often by Staphylococcus bacteria. In contrast to pimples, boils are deeper infections that can cause systemic symptoms like fever if they spread. Good hygiene and skin care can help prevent boils, and medical treatment may include draining and antibiotics when necessary.