If bugs get in and eat decomposing animals, what stops bugs from getting in and eating animals while they’re still alive?
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The presence of chemical defenses, such as toxins or repellents that are produced by the animal’s body. These can help to deter insects or make them less attractive as a food source. However, when they die, these defenses shut down and no longer work to keep insects out.
Also, many animals have behaviors that help to protect themselves from insects when they are living. For example, they may groom themselves regularly to remove insects from their fur, or they may use body movements or vocalizations to scare off insects that get too close.
Most bugs don’t burrow into living flesh. But a few do, and can absolutely eat parts of a living animal. One you’re almost certainly familiar with is the mosquito, but various insect parasites have historically been a big problem for humans and other animals until their eradication or control in most of the developed world. In the developing world, they still are, and insects like the botfly or worms like the [Guinea worm](https://en.wikipedia.org/wiki/Dracunculiasis) cause painful diseases.
The others are attracted to *decaying* flesh, which has already been broken down somewhat by cell death and the action of bacteria and fungi. That doesn’t usually happen while an animal is alive because (a) animals will actively move away from or brush off insects trying to eat them, (b) their immune systems are protecting them from bacterial or fungal growth, and (c) their cells are alive. (Immune systems do also fight larger parasites, though they’re less effective at it.) But even then, there are exceptions, and living animals like leeches are sometimes used in medicine even today to clean out necrotic (=dead) tissues from wounds.
I’ve seen a few animals that were infested with maggots while still alive. They’re usually wounded from another animal or something and are brought in to the vet as a stray. We kill the maggots with medication and I had to pick all the dead ones out of one (someone else did the others). 0/10 would not recommend.
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