Cats can be affected by a variety of diseases, including respiratory infections, heart disease, and cancer. The most fatal disease for cats is feline leukemia virus (FeLV), which is caused by a virus that can be spread through contact with an infected cat. Signs and symptoms of FeLV may include weight loss, poor appetite, and difficulty breathing. It is important to take your cat to the vet for regular checkups to help detect and prevent any potential diseases.