Question about apartment security deposit refunds?

First off, I need to say that I'm a very clean person. I have two cats and both are bathed once a month or so. I had a neighbor with a dog that he let outside without a leash all of the time and I think he gave us fleas...The apartment complex I lived in said that we had fleas and they charged us like 500 dollars or something crazy. They said they exterminated, but that makes me liable for the extermination charges (not to mention I know it wasn't my fault)? They said there was no evidence of fleas in any other apartments except mine, with my always indoor cats. I just can't understand how they can get away with this. I paid a 250 dollar refundable security deposit for the cats and a 250 dollar security deposit for the apartment, somehow I owe them 90 dollars. There was no damage to the apartment.