Appetite

• The primary sign that any animal is sick is when they stop. While stress or a minor illness may cause anorexia for a short time, if more than 24 hours pass without your cat eating, he should be seen by a veterinarian. It's also a good idea to monitor your cat's water consumption, especially if you think he is not feeling well. Some illnesses are indicated by the cat drinking more water than usual, while an illness that causes your cat to quit drinking can quickly lead to dehydration.
  
  

Litter Box

• What does or does not happen in the litter box is one of the primary indicators of your cat's overall health. Runny stool, hardened stool, diarrhea or lack of stool can indicate a host of gastrointestinal illnesses. Copious amounts of urine, little urine, dribbling or blood in the urine can indicate kidney and bladder diseases or infections. Your cat's behavior in the litter box may also indicate illness. If he is straining or crying in the litter box, he needs to see a veterinarian. If you cat suddenly decides to do his business somewhere besides the litter box, this is often a sign of illness.
  
  

Gums

• One of the best ways to determine if your cat is seriously ill is to look at his gums. A healthy cat's gums are pink. However, illness can change the color to white, blue, yellow or bright red, depending upon the condition. If you cat's gums have changed colors, he should see a veterinarian immediately. In many instances, this indicates a serious illness or emergency. White or pale gums often indicate anemia or systemic shock. Blue gums are a sign of breathing problem. Bright red gums may indicate septic shock or severe infection. Yellow gums are often a symptom of the loss of red blood cells, liver disease or gall bladder disease.
  
  

Respiratory

• Respiratory issues are common in cats. If your cat is coughing, sneezing, panting, has eye or nose discharge, or is wheezing, these are good indications that he has a respiratory infection and should see a veterinarian. Allergies may play a role in your cat's respiratory illness. Signs will include watery eyes and runny nose. You cat may also run a fever if he has a respiratory infection or allergy. A cat's normal body temperature is 100.5 to 102.5 degrees Fahrenheit. If your cat's ears feel warm, this may mean he is running a fever.
  
  

Personality

• If your cat's personality suddenly changes, this may be a good indication that something isn't quite right with his health. If your usually outgoing cat is suddenly sulking under the bed, or your usually aloof cat is seeking out attention, look for other signs or symptoms that he might be ill. Sometimes a sick cat will suddenly become aggressive, especially if the illness is causing him to be in pain.