Some of the facts in North America:
-There has been up to a 68% decline in the 20 most common birds since 1967. This decline is not solely from pet cats, but they do play an important part. Let's break it down.
-There are 30-80 million feral and stray cats, and 84 million owned cats
-A conservative estimate is that annually, free-roaming cats kill 2.4 BILLION birds in the Lower 48 states, 12.3 billion small mammals, and 650 million reptiles and amphibians.
The author makes a point that his pet cat may have killed 33 birds and dozen of mammals each year and caused the animals a very slow, painful death.
Relating to human health: "Cats are three to four times more likely than dogs to carry rabies, according to the Centers for Disease Control and Prevention. They also share many other parasites or infectious microbes with humans, including roundworms, hookworms, giardia and campylobacter." Cats also are the primary host of toxoplasmosis and pass it on to humans (almost 30% of humans worldwide have it).
For those of you who still want to let your cat enjoy the outside, I have two suggestions: Keep your cat on a leash (okay, I know, it sounds ridiculous but I have heard of cats who will go on leashed walks), or build/buy your cat an awesome, elaborate cage for your backyard, such as these ones. Otherwise, I'm sure your cat will still enjoy your company staying inside, and will remain cleaner, safer and disease-free.