Failure to disclose rodent infestation

Made offer on foreclosure. No infestation was visible at first viewing. 1 month later inspection revealed severe rodent infestation. Notified seller, requesting sanitation. Received noticed that he was working the issue. I then was put in a situation where I could not back out on the sale of my previous or new home. Seller now says contract states "as is" and that I closed on the house knowing about the infestation. Pest control found over 20 nests in basements, mouse blood and severe contamination due to mouse droppings in the entire house, basement, crawl space, vents, ducts, carpets, etc.. Contractors supposedly performed maintenance. They must have known, my neighbors know that previous owners already had rodent issues. The severity of infestation did not happen in the time I negotiated with seller. How do I proof failure to disclose? Is there any legal recourse that makes "as is" clause invalid since it was a severe infestation and made my house a hazardous environment. Thank you.