I'm writing a thesis for pathogens for infectious diseases. My essay is going to be about depressive disorder and anxiety disorder and I need to write a thesis for it.
Your thesis statement must include factual information plus your position/opinion/stance. Without your position on the topic, it isn't a true thesis statement. So think of this sentence as the angle you want to take on the topic and what you intend to prove by the end of your paper. (If your statement is simply factual, then there's nothing to prove!)
http://leo.stcloudstate.edu/acadwrite/thesistatement.html
This is one of the very best places I've seen online to help students write good thesis statements. It shows you sentences that aren't thesis statements and how to turn each one into real thesis statements.