The reason hair in food grosses us out is because the hair is perceived as being unclean and unappetizing when the hair is someone else's hair.
Hair in food can also be gross to us because the hair can be a biological or physical hazard and can indicate a lack of proper food hygiene.
Swallowing hair can also puncture the mucus membrane and hair can also pick up and carry bacteria, yeast, fungi and other microbes from the environment that the person is in or from their scalp.
Hair can also carry Staphylococcus bacteria that can cause hair loss.
If hair is in your food or remains in your food pathogenic bacteria on it could multiply and colonize food in the right conditions.
Swallowing a piece of hair is unlikely to harm you whether physically or through bacteria although it still is not recommended to eat food with hair in it.
I've found a piece of hair in my food before when eating out but I simply just removed the hair and checked for any other hairs and continued eating the food.
The small piece of hair is likely to not contaminate the food with bacteria but it could.