As the school year begins, many children might catch a stomach bug.

But, if the problem persists longer than the normal 24 to 48 hours, there may be an underlying problem: milk allergies or lactose intolerance.

Since both conditions have overlapping symptoms, parents often get the two confused. But they affect different systems in the body and call for different treatments.

Milk allergies reside in the immune system. When someone has a milk allergy, they are allergic to caseins and whey, the proteins found in cow’s milk. When they consume foods that contain these proteins, their immune system overreacts, causing allergic reactions that can lead to stomachaches, vomiting and diarrhea, typical lactose intolerance symptoms. But the reactions can also include wheezing, trouble breathing, coughing, hives, red spots, swelling or a rapid drop in blood pressure.

Dr. Mario E. Tano, a pediatric gastroenterologist with Joe DiMaggio Children’s Hospital in Hollywood, said milk allergies are more symptomatic in children and infants and should be closely monitored.

Although rare, children and adults with extreme milk allergies can experience a life-threatening reaction called anaphylaxis, which can lead to a state of shock and organ shutdown. Anyone diagnosed with a milk allergy should stay away from any foods with milk or milk proteins, such as casein, often used in cheeses, or whey, the watery liquid that separates from the solids in cheese making.

Parents should make a habit of reading the ingredients before giving food to their child. Even the slightest amount can cause a reaction in a child with allergies. For emergencies, doctors suggest children with food allergies keep an epinephrine pen with them at all times. If an allergic reaction occurs, a shot of epinephrine can stop it.

Dr. Alisa Muñiz Crim, a pediatric gastroenterologist at Miami Children’s Hospital, said many people mistake milk allergies with dairy allergies.

“There is a common misconception that if you’re allergic to milk, then you’re allergic to eggs, but that’s not true. You just have to avoid the proteins in milk, caseins and whey,” Muñiz Crim said.

Lactose intolerance has to do with the gastrointestinal tract and is less severe. It is more common among adolescents and adults and can be temporary. Lactose intolerance is the inability to digest lactose, a sugar that is found in milk and milk products. It is caused by a deficiency of an enzyme called lactate, which breaks down lactose into glucose and galactose, helping the bloodstream absorb the enzymes.

Children and adults who have lactose intolerance may experience symptoms when consuming milk or milk products, though they may be able to have small amounts depending on how severe their lactate deficiency is.

“Usually someone with lactose intolerance can tolerate about four ounces of milk a day, but it varies case by case,” Muñiz Crim said.

Symptoms typically last from 30 minutes to two hours and can range in severity depending on how much lactate the person’s body consumes. Tano said manmade enzymes such as lactate can help.

“For babies, there are formulas that are amino acid based and give them the calcium they need,” Tano said.

There are three types of lactate deficiency: primary, secondary and transient.