Disease mongering means companies or people spreading scary stories about normal things that happen to our bodies (like a cold or a headache) to make you think you're really sick and need medicine. They do this to sell more medicine and make more money.
Imagine going to a store with your mom for a candy bar. But instead of just selling you the candy bar, the store owner tells you that if you don't eat the candy bar, you're going to get really sick and need to go to the doctor. They even show you pictures of people who supposedly got sick from not eating candy bars. This is what disease mongers do - they exaggerate the bad things that might happen to you to make you want to buy something to fix it.
Sometimes, these stories are true and there really is a bad disease or illness that a medicine can cure, but usually they are not. So, it's important to be aware of these stories and do your research before buying any medicine or treatment. Don't believe everything you hear from companies or people trying to sell you things – always ask your doctor and make sure you really need the medicine before buying!