What Causes Hairless Patches On Head?

Hi my name is Fahad and I am currently living in germany. I am having a problem regarding my hairs on head. Right now there are two patches of my head are completely hairless from the roots and it has happened in past 1 or 2 weeks. Some body told me that its a disease I don't know how its called but in hindi or urdu language its named as "baal jhar". So what should I do?