Whether it is bacterium or fungus or protozoan or virus

It's caused by a parasite passed by the bite of a mosquito.

The Parasite Plasmodium can cause Malaria.

Read more here:
Malaria-Cause (http://www.webmd.com/a-to-z-guides/malaria-cause)

And here:

Plasmodium - Wikipedia, the free encyclopedia (http://en.wikipedia.org/wiki/Plasmodium)