Dragon Pox

All wizards and witches have to be vaccinated against dragon pox. The age for this inoculation is either at 5 or 16 years. As Muggle-borns only come into contact with the wizarding world at 11, a pureblood, who was vaccinated already at age 5, must take care of them while they fight off the disease. Of course, Draco is in charge of Harry.