Almost 3 years ago we got two cats. A female calico that was 2 years old, and a male (part siamese) that was 8 months. They lived together happily up until this weekend.
As the two cats were shedding quite profusely, I took each of them out onto the deck to give them a good brush. I started with the male and all was well, then put him back inside. Then took the female out for her turn.
I don't know if I made the mistake, but instead of carrying her in, I put her down on the deck so she could walk in through the back door. The male was at the back screen door, and he just fillped out, arch back and hissing fire and brimstone.
Now the two cats can't be anywhere near each other without the male just loosing it. He won't even let anyone near him if they have been near the female cat.
People are like a box of chocolates. It's hard to tell initially which ones are nuts.