Thanks everyone for giving me help, suggestions and advice. I now
learnt about Mildew (http://en.wikipedia.org/wiki/Mildew) and follow
(http://www.ext.vt.edu/pubs/housing/356-643/356-643.html).

My bathroom is tiny and has no windows and a tiny crappy extractor fan
that doesn't seem to do anything.

I will try and look for a good dehumidifier or/and normal fan to dry
the bathroom and lower the moisture. I already take off my shower doors
after a shower to let the insides dry.

Any more tips?