Sometimes I see people throw out an entire drink or bowl of food because a small bug was in it. Are they justified in doing this? Is there enough bacteria on common species of bugs (like ants, flies, beetles, dragonflies, caterpillars, moths, etc) to warrant any kind of health concern for a larger amount of food than what the bug was in contact with, or at all?
Well, first of all you should consider that insects and generally animals don't have so much hygiene, like us humans. Some of them, let's say the ants, yes some kind of hygiene is present, but not in the human degree. In fact, most insects are responsible for the most dangerous deseases in the world such as malaria , yellow fiver, sleeping sickness (espacially found in Africa because of the tsetse fly) and many kinds of encephalitis. This article claims that most of insects are carriers of deadly deseases. Also, about the second part of your question, you mentioned " If it is okay for bugs to touch food"... Well the answer is ofcourse no, because you don't know:
1. what the insect was touching before it "lands" to your food
2. If it has dropped any fecesto your food
3. And last but not least if the insect layed any kind of eggs to your food.
4. Some insects carry fungi that if a human being consumes them he might have serious problems, because some fungi are responsible for poisoning or infection.
The third one that I mentioned is responsible for the most cases of Tapeworm infection. This means that the eggs and larvae pass straight to your digestive system and create the infection. To sum up, unknown factors (like bugs) must not touch our food, because there is a chance of infection. To help you here's a wikipedia list that shows the most easily transmitted diseases to humans fron insects: List of diseases caused by insects.