In order to ensure you are getting a healthy kitten, please consider the following.
Kittens are born without an active immune system. Their immune protection comes from antibodies absorbed from first milk (colostrum) received from the mother. As antibodies received from the mother are depleted over the next six to eight weeks, the kittens own immune system continues to develop.
At about 8 weeks, it is believed that antibodies from the mother have been sufficiently depleted to challenge them with the kitten's first shots. However, since there is no way to know exactly when immunity from the mother has been depleted and the kittens own immune system is ready to be challenged, boosters should be given every 3 or 4 weeks until it reaches 16 weeks of age in order to insure immunity.
Many kittens are shipped at about the same time as maternal immunity is depleted and their own immune system is sufficiently developed to ward off disease or viruses. Stress caused by change of environment, a flight to a new home, the difference in temperatures, loss of littermates, etc., can and does often weaken a kitten's already delicate immune system further. When this happens, they becomes more susceptible to colds, or any latent virus that they may be harboring.
(Though the above article was written in reference to exotics, much of the article is applicable for vaccination of all animals.)
Certain viruses can be passed from the mother to the kitten, through grooming, use of the same cat box, or through the milk. Viruses can also spread to other cats by bites and fighting between cats. New research indicates that in rare instances, some can pass the placenta barrier.
*(There are many corona viruses, and not all mutate into FIP - see my Health & Diet Page for more information concerning the coronanvirus.)
It is always a good idea to make sure you do not bring a disease into your cattery. Some of the viruses can be debilitating, if not fatal - especially to kittens and older cats.
A good breeder should tell you if they have had any issues in their cattery and furnish you with information so you can make an informed decision concerning the purchase of a cat. A buyer also has the option to request specific tests be done prior to shipping. (The tests are usually at the buyer's expense)
Kittens can get worms through the mother's milk. Fleas not only attack their skin, but also cause tape worms. Ear mites are another issue that should be treated. (The use of wood litter that has been stored outside has been known to cause mite problems.) Internal and external parasites can weaken a kitten and even result in death.
Even if a cat comes to you with no viruses, diseases, or parasites, there are still genetic conditions that can shorten the life of the cat. You should ask if the parents carry any recessive, and/or inherent problems, and if there have been kittens from their lines diagnosed with genetic defects.
Genetics and hereditary conditions of pedigreed (purebred) and domestic cats and kittens
TUFTS Canine & Feline Breeding & Genetic Conference, 2005
The above is just a sample of the more common genetic diseases. There are others. Most breeders try to produce healthy cats, but it is always a good idea to be well informed and question a breeder in detail before you buy a cat.
There are also things which are not necessarily contagious or life threatening - but will keep your new kitten out of the show ring.
Feline Structure: Recognizing Normal and Abnormal
What to do if your Cat Produces a Deformed Kitten
If you are buying the cat as a breeder, you need to educate yourself about color genetics and be aware of any non-standard recessive colors or patterns that the cat may carry. *
*Color genetics can vary by breed.
It is also a good idea to read the standards for a cat before buying one for show or as a breeder. (Many standards also set out inherent problems that are common in their breed.)