cat fight dilemma
 

My cat is normally an indoor cat. She only goes outside with me & stays in the backyard. Our neighbours also have a cat & have recently allowed him to roam free. My problem is that my cat has been diagnosed FIP positive, and is not allowed to be with other cats. Unfortunately, my neighbour's cat roams into my yard and "fights" with my cat at the sliding glass door. Currently we are using a spray bottle aimed out of a window to discourage him. What do I do in the summer when there's only a screen door between them? Both cats have their claws and both are neutered. Both think that my backyard is their territory, and my neighbour's cat loves me as I look after him as well (another reason he hangs around). As my neighbour's cat has been an outdoor cat on a leash in the past and has fought with other roaming cats, can I assume that he may also be an FIP carrier?