As far as I know it is bacterial... the bacterial illness, scarlet fever, causes a distinctive pink-red rash, which occurs when the bacteria release toxins.
Scarlet fever is extremely contagious - people can catch it by breathing in the bacteria in airborne droplets that come from an infected individual's sneezes or coughs. Infection may also occur as a result of touching the skin of an infected person, or touching surfaces or objects that the infected person has touched.
Scarlet fever can accompany strep throat, which is a bacterial infection passed by people.
Poison ivy is a skin reaction to an oil in a plant.