In the german language light and easy is the same word, maybe also in romanian thats why there is the mistake probably.
There is no such thing as light flu. The English joke about man flu every time a man gets a cold but influenza or the flu is a VERY serious illness. It kills people in a weak condition. I was ill with it for 2 months and I have never known anything like it.
Ușor is the opposite of greu. The opposite of heavy is light. So I read this as "He has a light cold."