The illness was what they called it. It came and went, no one knew how it was spread but it affected random people here and there. It changed the age of people, some people were lucky and were made younger however sometimes it could take you right down to when you were a baby, you still had the same mental capacity but you had to learn how to walk and everything. Others were affected by becoming older, several deaths had been caused by the illness, poor souls, and some were only 21 and died at the age 185 the same day. People who had the money fled England and waited till it died down, others carried on like normal hoping they wouldn't be the next victim of the illness.
One of the many people affected with this wizarding illness was the great potions master Severus Snape, who was de-aged to the tender age of 10. Dumbledore became his legal guardian and privately tutored him.
Ten years later there hadn't been a case of the illness for over 5years and people deemed it safe. The ministry changed the laws so that if you were infected by the illness your year of birth was changed depending on the age you were de-aged to, so you became that age legally.
Harry Potter the savior of the Wizarding World had been travelling the world whilst the illness was around so was not affected, the DADA teacher at Hogwarts had been fired due to being an utter idiot so Harry took the job. Severus Snape now 20 years who had taken a master in Potions when he was only technically 16, had been back for a year as the Potions Master.