Nearly all infectious diseases of cats are caused by viruses.  The best prevention against viruses is vaccination, and the below viruses are the most common that you should understand and make sure your cat is vaccinated for.  Discuss this group of shots with your vet so that they're taken care of at the right time and in the right order.

Feline Infectious Enteritis (Feline Panleukopenia) (FIE)
This viral disease is highly contagious.  The virus attacks the gut and the white blood cells.  Signs of FIE include depression, loss of appetite, and vomiting.

Cat Flu
Feline Respiratory Viruses (FCV) and Feline Viral Rhinotracteitis (FVR) are two viruses that affect the upper respiratory tract, FVR being a herpes virus and usually the most serious.  Cat flu is most dangerous in kittens and elderly cats and if not treated can result in death.  While it is also serious in a healthy adult cat, they have a better stamina to recover.  Symptoms include coughing and sneezing, runny discharge from eyes and nose (which becomes thicker as the disease progresses), and loss of appetite.  Cats exposed to the virus before being vaccinated can become carriers and should be isolated from other felines.  Chlmaydia is also a virus, which tends to cause more of a conjunctivitis than the traditional cat flu.

Feline Leukaemia Virus (FeLV)
Often referred to as feline aids, this virus can cause cancer of white blood cells and the lymph system years after the original infection has occured.  It may also break down a cat's immune system or cause anaemia.  The FeLV virus is generally transmitted by long exposure to an infected cat's saliva and/or blood.  Syptoms include weight loss, vomiting, diarrhea, and anaemia.  A cat will not die of FeLV and may live for many years, however it can suffer from illness due to the breakdown of the immune system.

Rabies
Rabies is a deadly diseaseich unlike most other cat diseases can be passed to humans.  Early systems of this virus include a quiet cat becoming aggressive.  The final stage is paralysis and coma which ends in death.  Once the cat has contracted rabies little can be done.  Your cat should be up-to-date with its rabies vaccinations at all times, especially if it is an outdoor cat.

Feline Immuno Deficiency Virus (FIV)
FIV is similar to the HIV virus in cats, and human cannot catch FIV from cats.  FIV breaks down the immune system of a cat leaving it susceptible to infection.  The virus is spread through the saliva of an infected cat.  At first the cat may seem a bit off and then may develop a secondary infection.

Feline Infectious Peritonitis (FIP)
There are two kinds of FIP, wet and dry.  Wet will cause a swollen abdomen, which may result in poor health, diarrhea, and vomiting.  The dry virus affects the nervous system and the symptoms are harder to detect.  They include jaundice and loss of coordination.  This virus is mainly transmitted in the cat's saliva.  An infected cat must be isolated to prevent further spread of the disease, and no vaccination is available.