First it was lead paint that was a huge potential issue (and liability) for landlords. Now the current hot topic seems to be mold. Even though most mold isn’t toxic, insurance companies are so spooked out about it that they have clauses to exclude mold related issues either entirely, or at least past a certain threshold.
Landlords and property managers should familiarize themselves with potential mold related issues, testing and remediation. The EPA has an entire section of their site dedicated to this issue. You can find it here: http://www.epa.gov/mold/
The EPA also publishes A Brief Guide to Mold, Moisture and Your Home: http://www.epa.gov/mold/pdfs/moldguide.pdf