I think my cat/kitten is sick...now what?

Here is a quick list of ailments; this is by far not a complete list and by no means should it substitute a vet's advice. This is only a little peek at what sort of things your cat will be vulnerable to, so take this into consideration when getting an animal--can you pay for vet bills and the necessary check-ups?
(What sorts of vaccinations will the cat need? Why should I vaccinate my cat?)

eye infection
(conjunctivits)
Eye infections are quite common in cats, even indoor kitties can contract them from time to time. They're easy to treat, but if they get out of hand the cat may lose the sight in one or both eyes. Eye infections, like conjunctivitis (pink eye) in humans, are contagious and need to be monitored. If you notice your cat is closing one eye, if the eyes are runny or weepy and have a greenish/yellowish goop coming from them, make an appointment with your vet. S/he'll give you an antibiotic ointment and will tell you how to administer it. Make sure you complete the treatment for the recommended number of days!
fading kitten syndrome
This is a catch-all phrase for kittens that just fail to thrive. Causes are unknown, but possibilities include organ failure due to unseen birth defect or an undiagnosed ailment. Kittens may have trouble keeping down food, they may become seriously dehydrated due to severe diarrhea, or they may just "fade", even with treatment. For vomiting or diarrhea, try feeding the kitten a bland diet of plain boiled chicken and boiled white rice, then gradually introduce their regular food mixed with a little water after their stool returns to normal. Make sure the kitten is drinking water. If it persists for more than a day, make an appointment with the vet.
feline leukemia virus disease complex
(felv)
This is the leading cause of death in household cats (second to trauma). Initial illness lasts 2-16 weeks. Symptoms include fever, apathy, loss of appetite, weight loss, constipation, vomiting, and diarrhea. Some cats might develop enlarged lymph nodes or anemia. Every exposed and infected cat has three possible outcomes (each as likely as another): 1) This lasts for less than 12 weeks, and the cats develop an antibody which cures them of the disease, they cannot transmit the disease, and have a normal life expectancy; 2) The virus remains present for over 12 weeks, and the cats become susceptible to other FeLV-related illnesses which are invariably fatal (about 50% die within 6 months and 80% within 3.5 years); 3) These cats develop a latent infection that eliminates the virus from the blood and saliva but doesn't extinguish it completely. The incidence of latent infection after 3 years is quite low, though it may recurr during periods of stress or concurrent illness. These cats are susceptible to FeLV-related diseases. You should have your cat vaccinated and tested often, and annual boosters are needed. It's recommended that FeLV-positive cats should be removed from a mulitple-cat household, and that they should not come into contact with women of child-bearing age and individuals with immune deficiency diseases (although it's never been reported to have been transmitted from cat to human, lab tests show it's possible).
feline immunodeficiency virus
(fiv)
This virus was first discovered in Northern California in 1986, and is related to the AIDS (HIV) virus in humans. However, it cannot be given to humans, just as HIV cannot be given to cats; it's species specific! Incidence is highest in outdoor male cats between 5-10 years, which suggests transmission by biting. Symptoms include fever, swelling of the lymph nodes, diarrhea, skin infections and anemia. There is a latent period of several months to 3 years, after which appear gradually again and progress slowly. Cats might also exhibit signs of unexplained ill health, like loss of appetite, emaciation, mouth and gum disease, long-standing diarrhea, etc. There is no vaccine, but prevent infection by keeping cats from roaming or fighting with infected strays. Neutering your cats can reduce the incidence of fighting. Cats with FIV should be isolated from other cats.
feline infectious peritonitis
(fip)
This disease tends to affect kittens, cats 6 months to 5 years, and cats over 11 years old. There are two forms (both secondary to a milder form of FIP, which is like URI--less than 5% go on to develop a secondary form), wet and dry or disseminated (disseminated form is more difficult to diagnose), and both are fatal. Symptoms include loss of appetite, weight loss, listlessness, and depression. Jaundice may appear (a sign of liver failure), as well as dehydration, vomiting and diarrhea. Cats that have developed one of these secondary forms will die in a few weeks, and should be kept from other cats and made as comfortable as possible.
feline
panleukopenia
(feline distemper)
Distemper (bears no relation to canine distemper) is common with outdoor cats, although indoor cats can get it too if exposed to other cats with it, or through your clothes (if you hold a cat with distemper then go pet your kitty without changing, etc.). It's an airborne, extremely contagious disease that, if caught quite early, can be treated; without treatment the cat will never recover completely, may suffer severe neurological damage, and in most cases, is fatal. Kittens with distemper may appear lethargic, will stop eating/drinking, and may vomit or have diarrhea. It can be prevented with vaccinations. Distemper is a hardy virus. If your kitty has been exposed or has contracted the disease, do not take any other cats into your home for more than a year. Scrub every available surface with a 1:32 dilution of bleach in water. td>
rabies
Rabies is a fatal disease that can be transmitted to humans, usually by a bite from an infected animal. Cats living outdoors in rural areas are at highest risk. The virus is carried through the saliva, and the incubation period can be from 9 to 60 days. At first, symptoms are subtle and may include only personality changes. Affectionate, sociable cats may become aggressive and irritable. Shy cats may become more affectionate and outgoing. Oftentimes they avoid light and go into hiding. There are two forms: the so-called furious form and the paralytic form. Cats may show signs of both. The furious form includes the stereotypical "mad dog" characteristics: springing up and attacking people about the face and neck, muscle twitching, tremours, and violent convulsions. The paralytic form, occurring in 30% of the cases, slowly paralyses the cat. The swallowing muscles in its throat will become paralysed, and the cat may drool and paw at its mouth. As encephalitis progrsses, the cat loses control of its rear legs and collapses, unable to get up. Death from respiratory arrest occurs 1 or 2 days later. There is no effective treatment for cats, so be sure to vaccinate your kitty!! If bitten by a wild animal, unprovoked or not, immediately clean the wound and the surrounding area. This greatly reduces the risk of rabies. DO NOT handle, pet, or medically assist animals suspected of having rabies: this could only serve to spread infection. Call animal control and have your pet impounded immediately if you suspect it has contracted rabies.
upper respiratory infection
(uri)
This is a fairly common disease too, but luckily it's much easier to treat (treated with antibiotics). Symptoms include runny or weepy eyes, wetness around the nostrils, and sneezing. Cats rarely sneeze out of the blue, unless there's a lot of dust in the air, but if you hear your cat sneezing and there's seemingly no reason, call your vet for an appointment. You may also notice the presence of a white eyelid in the corner of his/her eyes. The cat may also feel warmer than usual (indicating a fever), and s/he may refuse food and water and lie around listlessly. It's imperative that they do not get dehydrated! In bad cases, you can buy a bottle of Pedialyte in the infant section of the grocery store (get the unflavoured kind), and mix a little in with the cat's water bowl. You could also take a needleless syringe and squirt some into their mouth; two or three syringe-fulls every couple of hours should be sufficient. But do this only with the vet's ok; if your cat is on other medication it could be harmful or ineffective. This is contagious, so keep other cats away and wash your hands after handling him/her.

Tips for bottle babies: