This is caused by two important viruses and may be complicated by secondary bacteria. The two viruses are called feline herpesvirus and feline calicivirus, and together they form the disease commonly called "cat ´flu".
Feline herpesvirus will infect most cats in their lifetime, and most cats will become lifelong carriers. They may excrete the virus when they become stressed or ill, causing repeated bouts of illness.
Vaccination protects cats from disease, but the immunity does not last long and needs regular boosters for the best possible protection.
The virus attacks the eyes, mouth and lungs, causing severe symptoms such as fever, eye ulcers and pneumonia. The infection is often made worse by secondary bacterial infections. Infected mothers give birth to small, weak kittens.
Feline calicivirus is also very common. It is generally less severe, but causes painful ulcers of the mouth and tongue, and may again be complicated by bacterial infections. Vaccination is highly effective at protecting cats from disease, but regular boosters are required.