What happens if a fly/mosquito/whatever harmful insect flies through your mouth and into your stomach?

I used to read a fictional story about a person who had a mosquito fly into their stomach, and yup, it took a big surgery to finally rescue them. The doctor said that the mosquito could “cause death since it bites the internal organs and leaves serious infection/diseases” and could “live and drain blood from the inside”.

Well, that was just one made-up story. I wonder what would happen in real life, if something similar happened. Would the insect cause death, or would they be dead themselves inside our body?