Children love sweets (a fair portion of children, at least). There’s just one sweet that the smaller and younger members of that crowd are strictly advised to never consume: honey. The World Health Organization is just one of many medical groups worldwide that agree that honey should not be fed to infants under 12 months of age, whether added to their formula, used to soothe teething, or employed to sweeten foods. But why shouldn’t babies eat honey?

The problem with feeding honey to babies stems from their developing microbiome—the array of microorganisms, such as bacteria and archaea, that live inside and on humans and that assist in the regulation of diet and affect many bodily functions. Before babies reach the age of one year, the bacteria in their gut is not developed enough to stop certain harmful bacteria from infecting the body. Clostridium botulinum is a potentially poisonous bacterium that, in addition to causing the illness known as infant botulism, is often found in raw and processed honey. According to a study published in 1998, up to 25% of honey products have been found to contain spores of that bacterium.

Symptoms of infant botulism can be scary. They include a weakening of the muscles, causing a child to appear limp and unable to move arms or legs, and potential respiratory failure. There are no long-term effects of the illness, however, and treatments for life-threatening symptoms are quickly available in severe cases. Most patients make a swift recovery after receiving an antitoxin, but the best way to avoid infection is to avoid the number one cause: honey.