The best way to avoid this is to remove everything from the lunch bag whether it's right after work or after school. We suggest using a slightly damp towel with a tiny drop of mild detergent, wipe the inside and outside, then allow to air dry. But in the case that Friday's lunch may still be in the bag until Sunday night or Monday morning and this includes a half eaten egg salad sandwich or worse, you're going to want to try this easy fix:
Take a slice of bread or a paper towel and moisten until it's completely damp but not dripping with white household vinegar (preferably distilled). Place it in your lunch bag, close the bag and leave overnight. In the morning, remove and discard the paper towel or slice of bread and wipe the inside of the lunch bag with a clean, damp towel and allow to air dry.
Another option is to sprinkle baking soda inside the lunch bag and stuff with crumpled newspaper to absorb the moisture and smell. Close the bag and after 1 or 2 days, remove and discard the newspaper and baking soda. Wipe the inside of the lunch bag with a clean, damp towel and allow to air dry.