Vomiting and diarrhea are mechanisms that help the body eliminate substances perceived as harmful, such as viruses or bacteria. Nausea is the sensation that occurs before vomiting and often occurs with diarrhea and other GI symptoms. There are many possible causes for nausea and diarrhea, some of which can lead to complications if left untreated.

Infection

Viral and bacterial infection occurs when pathogens enter the digestive tract and multiply. Bacterial infection often stems from food poisoning, while viral infection--or gastroenteritis--is caused by contagious viruses like rotavirus and adenovirus. Both bacterial and viral infections can cause nausea, diarrhea and other symptoms like vomiting, fever and weakness.

Lactose Intolerance

Lactose intolerance is the inability of the intestine to absorb and digest lactose. Ingestion of foods containing milk can cause nausea, vomiting or diarrhea in lactose intolerant persons, according to WebMD. Other defining characteristics include excessive flatulence, abdominal pain and bloating.

Irritable Bowel Syndrome

Irritable bowel syndrome (IBS) is a constellation of symptoms rather than a disease. Symptoms involve the digestive tract and may include nausea, diarrhea, constipation, vomiting and general gastrointestinal upset. While the cause isn't known, factors like stress, hormonal fluctuations and food sensitivities can trigger symptoms in some sufferers, according to eMedicineHealth.

Inflammatory Bowel Disease

Inflammatory bowel diseases, or IBDs, are autoimmune conditions that cause inflammation in the digestive tract. Symptoms primarily affect the intestines and include nausea, vomiting, diarrhea, weight loss, fever and abdominal pain. Celiac disease, Crohn's disease, ulcerative colitis and diverticulitis fall under this category.

Warning

Diarrhea can lead to dangerous complications such as dehydration and electrolyte imbalance. If you experience severe diarrhea accompanied by vomiting, blood in stool or fever higher than 102 degrees, seek medical help immediately.

Source:

WebMD: Lactose Intolerance - Symptoms

eMedicineHealth: Irritable Bowel Syndrome

More Information:

Mayo Clinic: Diarrhea: Causes