Disease.
Let's get right to it. Feline Leukemia Virus (FeLV), Feline Immunodeficiency Virus (FIV), Feline Enteritis (FeOV which can lead to Feline Infectious Peritonitis), Feline Panluekopenia (FP or feline distemper)  any of which will probably result in either a quick or slow death for your cat are passed primarily from cat to cat. Some cats don't die, instead they become carriers who spread the viruses. The more cats your cat has contact with, the more likely one of these common deadly diseases will shorten his life.  Some of these diseases have inoculations to provide some protection.  However, most only provide from 10%(FIP)-70%(FeLV) protection, and then only if your cat is current on his inoculations and boosters. To put things into perspective for new cat owners, we left rabies down here. Rabies has no where near the impact on cat populations compared to the above deadly diseases.

Less Serious Diseases
Need more diseases? These are less likely to kill kitty, but can if not treated soon enough. Coccidia, Toxosplasmosis, Feline Viral Rhinotracheitis, upper respiratory infections.

"Cats are both germ bags and germ magnets.  
Do you want to send your germ magnet out to be where the germ bags are?"

Parasites.
Outdoor cats suffer to a much larger degree than indoor cats from fleas, ticks, ear mites, worms, and funguses (like ring worm). Separation from other animals along with the house itself providing shelter protect to a higher degree.

Monitoring Urinary Tract and Bowel Health.
One of the best indicators of feline health is to monitor the efforts, difficulties and results of your felines litter box efforts. Straining is a sign that kitty needs to see the Vet to be checked for urinary crystals, blockages or stones. Color, presence of diarrhea, or straining provide important indicators of health problems. Observing and then taking advantage of the provided evidence by scheduling a Vet appointment can add years to the life of your pet.

Road Kill.
In some parts of the country vehicles account for the largest reason for death of adult cats that are allowed to roam outdoors.  Warm tires and radiators provide enticing heaters for cats when the weather is cold. Maybe your kitty will get lucky and only need to see the Vet for six weeks 

Poisons.
Do you know that just a few laps of the tongue in antifreeze coolant that leaks in your garage can kill your cat? Just imagine how much is in your curb. It doesn't take much more to quickly put down a dog. Even if your Vet's heroic efforts keep your pet alive, you can anticipate organ failure to begin soon after the antifreeze is ingested.  Other poisons include snail bait, lawn chemicals, bait left out to kill rodents and even some plants commonly found in our local landscapes.

Other Animals.
Fights with other cats, dogs and animals can cause serious injury.  Cats develop abscess infections at the sites of wounds or bites. These infections require veterinary care to help fight the infection and not impact the cats immune system. It's not just Toms cats, females get into cat fights too. Many of the fights are territorial. Tom neither knows nor cares that Fluffy Boy was neutered.  

Excessive Sun
Felines obtain vitamin D from their diets. Unlike you and I, cats do not produce vitamin D when sun hits their skin. Excessive sun can bring on squamous cell carcinoma, a fast spreading malignant cancer that affects the ears, nose and mouths of cats.

$$$ Do You Like to Save Money?
By looking at the items above, you can see that over the years you should save on medical expenses related to your cat. Not only could you save from less disease and injury, many veterinarians and the American Association of Feline Practitioners do not recommend some of the inoculations if they know a cat will always stay inside. Over the years, you should have no problem offsetting kitty litter costs.

$$$ Do You Like to Save More Money?
You won't have to bail your cat out of the shelter. For now, you'll also have to put enough gas in the car to drive to Orange to pick up your pet.

Pet Overpopulation.
Are you adding to the pet overpopulation problem by allowing an unaltered pet to roam……?

Your Cat Will Probably Live Much, Much Longer.
Estimates of the average lifespan of free roaming felines in an area like ours is 2 to 4 years. Estimates of the life for a cat that is not allowed outdoors range from 14 to 16 years. When protected from harm, it is not uncommon for cats to live for more than twenty years. It is much more stressful on a cat to be outdoors dodging truck tires, car fans, aggressive animals and human danger than to be home watching through the window in between all of those naps.

You Might Get Flowers from the Lady on The Corner.
You may find that your neighbors think more highly of you, especially when you comment on how nice their garden grows without Fluffy burying his stuff.  Both you and your neighbors can awake refreshed and won't know the melody and words from memory of your cats favorite 2 A .M. operas.

You Will Hear More Birds Sing.
San Juan Capistrano promotes leaving birds to sing, sing, sing. Cats are skilled hunters and can significantly impact the local native bird populations. Join kitty at the kitchen window and watch and hear their joyful songs. A bird feeder visible from a window will entertain him, without putting the birds at risk. Both birds and rodents are transmitters of intestinal parasites that cats pick up when they eat their prey.

Lost and Not Found.
The recovery rate for lost cats at shelters is unfortunately very dismal. Many people wait a week for their cat to return from his outing. Unfortunately, there are times when this is too long. All cats and dogs, even indoor cats need to wear identification. Locally, either a moderate earthquake or our recurring 50-70 mile per hour gusts of Santa Ana Winds can cause broken windows. During natural disasters cats and dogs become frightened and they can flee their safe home.  People recognize that a cat wearing a collar is someone's pet and not a stray.  They will spend much more time trying to coax the animal to safety and then locate the owner.  In our area, most abandoned cats, live very sort lives outside.

Last But Not Least 
Your Cat's Biggest Problem Outside.
Here in South Coastal Orange County and inland, coyotes present the largest threat to outdoor cats and small dogs. Don't be fooled. Coyotes do walk the beach in Dana Point or Capistrano Beach.  They can be seen touring the local apartment complexes, looking for an easy meal. Easy means domesticated, as in cat or dog. San Juan Capistrano residents enjoy the open spaces and ridgeline protections that allow us to feel the openness of the beautiful area around us.  Each of us hears the nightly howling of adults and the yelps when training of the coyote pups occurs. Our natural creeks and their tributaries, railroad beds, bike trails and roads provide fast access to any part of town. Think your pet is safe as long as you bring it indoors after dark? Think again. Coyotes hunt when they want to eat. Attacks on animals occur 24 hours a day. Coyotes appear to be adjusting and becoming less concerned about people. A July 2001 afternoon attack on a four year old boy (he's fine now) occurred in a residential Irvine park.  In October 2001, three youngsters were attacked during their lunch period at a San Clemente elementary school. No reason exists to think that this coyote or his cousin Wiley would not have jumped your short five foot wall to get to your pet.

In the article covering the October attack "Terri Hunt said that during the last 10 years in San Clemente, her family has lost eight cats to coyotes".  "We've given up on cats" she said. 

Unfortunately, the coyotes have not given up on the cats or small dogs.
Keep your cats and small dogs inside.