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Pyothorax is when pus fills the chest and makes it very hard for breathing. It is a condition and not a disease. If it was indeed pyothorax it most likely from an old bite wound or other penetrating type wound to the chest, introducing bacteria in the chest cavity. Treatment involves chest tubes, antibiotics, sometimes surgery, and flushing out the chest cavity. It is an extremely costly treatment with a fair prognosis.
Now if there was fluid in the chest that was not PYOTHORAX but transudate or chylous ( your vet would have said) the cause of that build up is different.
Because pyothorax is usually from a wound of some kind, I doubt you have to worry about other cats developing the same problem.
I am sorry for your loss,