Distemper is one of the worst, and most common of cat health problems. It is caused by a parvo virus and is similar to that found in dogs.
This is an extremely deadly disease, highly contagious among cats, and the virus itself can live in the environment for years. If planning to bring a new cat into your home, be sure it has been vaccinated first.
The disease has a high mortality rate - up to 60 percent of adults, and 90 percent of kittens. The incubation period is 3 to 7 days. The disease begins with a fever, loss of appetite, weakness and depression. Diarrhea and vomiting are common. And while they remain thirsty, victims usually suffer dehydration. Once infected, they seldom live longer than 5 or 6 days. If they survive, they usually have some brain damage, which results in a lack of coordination later.
The only good thing that ever comes of this is that survivors are solidly immune for life and never need to be vaccinated against distemper.
Because some cats may experience severe vaccine reactions and can die from that, it is best to wait until kittens are old enough, strong enough, and healthy enough to tolerate vaccination.
Check with your vet on that. It's usually around 9 weeks, but it isn't always best to follow a schedule from a book, but rather, to be sure individual kittens can handle the vaccine.
To be safe, never vaccinate a kitten that is still nursing, no matter what age it is. The kitten is receiving passive immunity from the mother and the vaccine is wasted anyway, as the kitten will not develop its own immunity as desired.
Also be sure to have any vaccinations completed at least 2 weeks prior to any potential exposure (no matter how remote the possibility), such as what could occur if going to a farm, to a new home, a cat show, or anywhere that groups of unacquainted, unrelated cats may be present.
More information here: The Problem Cat - Diseases