One could say that a pimple is a whitehead that has gone wrong! Indeed, sometimes a bacterium found naturally on the surface of your skin (Proponibacterium acnes) settles into your blackheads and multiplies: it thrives in this fatty oxygen-free environment and it will proliferate very quickly. So your whitehead swells and becomes red and painful: you end up with a pimple that is red or white depending on the case.