Accidentally eating a piece of moldy food can turn a "look at me eating leftovers like an adult" moment into a "holy shit I've got food poisoning" moment pretty quickly, but is consuming mold really all that bad for you?
The short answer is no, you're probably not going to die from eating mold; you'll digest it like any other food, and as long as you've got a relatively healthy immune system, the most you'll experience is some nausea or vomiting due to the taste/idea of what you've just eaten. That said, this stuff is definitely not good for you, and if you're allergic to mold, it can bring on uncomfortable symptoms including irritation of the eyes, nose, and throat.
What's more, although common bread mold isn't especially harmful, there are some dangerous molds out there which can be detrimental to your health. In fact, a few of them can produce poisonous substances known as mycotoxins, the nastiest of which (aflatoxin) can cause cancer according to the USDA. That said, contamination tends to occur farther up the chain than your kitchen, primarily appearing in grain and nut crops, so your regular household mold probably isn't aflatoxin-related.
Can't you just cut the mold off your food?
Although it might be tempting to simply tear off the moldy bits in the interest of not wasting food (or money), there's no guarantee you're actually removing the whole fungal infection by doing so. Mold's thread-like roots can reach deep into the food in question, so simply removing the obviously nasty parts won't do. Better to err on the side of cutting off too much than too little -- or better yet, just toss the whole thing out. To clear things up, the USDA has a food-handling chart outlining which foods can safely be pruned, and which should just be discarded.
Doesn't toasting your bread kill the mold?
No, actually, not necessarily: although you can technically kill mold by exposing it to high temperatures, you probably won't reach those temperatures without burning the ever-living shit out of your bread -- and even then, there's no guaranteeing it worked. Plus, even if you don't mind the taste of charred bread, you'll likely still have a lingering mold flavor on your toast. Why not just get fresher bread?
What about moldy cheese?
Some cheeses (like Gorgonzola) do have mold cultures purposefully added to them, although these specific molds are actually unable to produce mycotoxins. Setting aside those intentionally infected cheeses, different types of cheese will require different handling when mold is present: moldy hard and semi-soft cheeses can be trimmed without worry, but moldy soft cheeses should be scrapped entirely.
How can you prevent mold from forming?
Simple steps to avoid mold formation include securely covering foods in the fridge (with plastic wrap or containers), eating leftovers within four days, and not leaving perishable food out for more than two hours. Most important though, is to be realistic about how much food you eat during the week: if you live alone, you're probably not going to finish a whole loaf of bread before mold shows up.