This can vary from a few mild signs to something very severe. The strain will determine the symptoms. The healthier a cat is, the better the natural immune system can work to fight infection. The same amount of infective virus that would make an unhealthy cat very ill may just depress and show mild signs in a healthy individual. The better the state of nutrition, the better a cat can fight the disease.
All cats have an inherent immune system.
It is difficult to explain how this relates to disease, but it has been proven that certain purebred cats are more prone to some diseases than others. The larger the population in a given environment, the more susceptible these cats will be to respiratory disease. Once a cat or kitten has been infected with Calici virus, it will continue to spread it into the environment and to other cats even after recovery. There is no time limit to this, and it can continue for a few short weeks to as long as a few years. All cats must come in contact with the virus to become infected.