Stomach Flu (Bug) Causes, Duration, Symptoms, Treatment

Every person is affected by the stomach flu at some time in life. It tends to be more common among children but adults are also at risk.This type of flu is known as viral gastroenteritis. It is caused by a virus which is one type of ‘bug’. However, there are other stomach bugs apart from viruses that can also cause gastroenteritis.

Most of the time the stomach flu and other stomach bugs have a short term effect on the stomach and bowels.After 2 to 3 days of intense nausea, vomiting and diarrhea, the infection resolves and the symptoms slowly subside. However, in some cases these infections can be severe, require medical treatment and even lead to deadly complications.

What is a stomach bug?

A stomach bug is infective gastroenteritis caused by bacteria, protozoa or viruses. These bugs enter the gut and then inflame the stomach and intestines (small and large) which results in diarrhea, vomiting and other symptoms. We also refer to this as food poisoning when it arises from consuming contaminated food or water. Most of these bugs cause a diarrheal illness but some may only cause vomiting with little to no diarrhea.

Another term commonly used for gastroenteritis is stomach or tummy flu which refers to viral gastroenteritis. This often occurs in seasonal episodes and can affect a number of people in the same environment (air borne) but it is not due to the infection with the influenza virus. Several viruses can cause viral gastroenteritis, with rotavirus being one of the most common among children.

Read more on food poisoning vs stomach flu.

Causes, Incubation Period and Duration

The bacteria, viruses and protozoa that are known as stomach bugs cause infective gastroenteritis. Illness does not immediately develop upon contact. It can take days or even weeks from the time the bug enters the body for symptoms arise. This time where there are no symptoms is known as the incubation period. The disease may then last (duration) for days, weeks or even months.


Incubation period less than 6 hours and duration of 1 to 2 days due to action of toxins:

  • Bacillus cereus
  • Clostridium spp. (C.perfringens)
  • Staphylococcus aureus

Incubation period 12 to 72 hours and duration less than 8 days :

  • Campylobacter spp
  • Clostridium difficile (variable duration)
  • Escherichia coli – different serotypes
  • Enterotoxigenic E.coli (ETEC) causes gastroenteritis due to the action of the enterotoxin.
  • Enterohemorrhagic E.coli (EHEC/Shiga toxin-producing E.coli) causes gastroeneteritis due to the action of verotoxins plus directly invade the mucosa of the small intestine and colon.
  • Enteroinvasive E.coli (EIEC) invades and destroys the colonic mucosal cells.
  • Salmonella spp
  • Shigella spp
  • Vibrio cholerae


Incubation period of 18 to 48 hours and duration less than 1 week :

  • Rotavirus
  • Norovirus

These are the most common but astroviruses, adenoviruses and sapoviruses may also cause viral gastroenteritis. Acute viral hepatitis should also be considered in a case of nausea and vomiting with other flu-like symptoms.


Incubation period of 1 to 4 weeks :

  • Amebiasis (UK ~ Amoebiasis) – duration 1 to 4 weeks
  • Cryptosporidiosis – duration 5 days to 4 weeks
  • Giardiasis – duration 3 to 10 weeks
  • Isosporiasis – duration is variable
  • Microsporidiosis – duration is variable

Isosporiasis and microsporidiosis is more frequently seen in cases of HIV/AIDS and duration depends on the patient’s immune status, accessibility to HAART and management. Persistent (>14 days) or chronic diarrhea (> 30 days), especially in adults, due to cryptosporidiosis should also raise the concern about HIV/AIDS.

Signs and Symptoms

The common signs and symptoms include :

  • Nausea: Constant and does not always ease after vomiting.Retching is the involuntary but unproductive effort to vomit.
  • Heartburn and chest pain may be present as gastric contents rise up the esophagus coupled with muscle spasm from retching.
  • Vomiting: Severe with recurrent retching after expulsion. May settle in the latter stages of the infection although nausea may still be present. Eating solid foods usually aggravates nausea and vomiting. Bloody vomit (hematemesis) is not always present and may only occur with persistent or violent vomiting (refer to gastritis caused by a Mallory-Weiss tear).
  • Diarrhea: Frequent, watery stools (large volume) often accompanied by intestinal cramps. Certain infections and persistent diarrhea may result in bloody stools (melena ~ black, tarry; hematochezia ~ red, bloody). Diarrhea may be mild in some infections. Constant urging (tenesmus) may persist between evacuations.
  • Abdominal pain and abdominal discomfort: Ranging from a dull ache, to colicky or severe abdominal pain. Bloating and tenderness often present, especially in bacterial infections.
  • Fever: Fever may be absent in some cases of infectious gastroenteritis – bacterial toxins may not cause a fever.
    Other microorganism may cause a low, moderate or high fever. Varies from >100F/37.7C to 104F/40C – temperatures in excess of 104F/40C requires immediate medical attention.
  • Dehydration: Complication of fluid loss – vomiting, diarrhea, perspiration. May be moderate to severe and may lead to death if left untreated. Dizziness, dry mouth, poor skin turgor and fainting. Signs of shock due to severe dehydration – immediate medical attention necessary.

Treatment of Stomach Bugs

The focus should be on management – adequate rehydration, bed rest and gradual introduction to solid food. Acute infectious gastroenteritis should settle in 2 to 3 days in a healthy person. Medical treatment should be sought if the symptoms are persisting for more than 5 days or if there is any sign of moderate to severe dehydration.

  • Oral rehydration therapy is necessary to reduce and prevent dehydration. In severe cases with constant vomiting, oral rehydration may not be possible. Therefore IV rehydration may be necessary. Solid foods should be consumed once the vomiting subsides. The BRAT diet may be better tolerated initially.
  • Antibiotics may be necessary for treating infections caused by Vibrio cholerae, Shigella spp and salmenollosis. For other bacterial causes of gastroenteritis, antibiotics should be avoided unless a person is immunocompromised or in a case of persistent diarrhea where a stool culture confirms the bacterial species.
  • Antidiarrheal agents are not advisable in acute infectious gastroenteritis. The use of OTC (over-the-counter) antidiarrheal agents often results in complications especially if used excessively in a case of bacterial gastroenteritis.
  • Probiotics containing Saccharomyces boulardii and Lactobacillus casei GG may assist with persistent diarrhea following a case of acute infectious gastroenteritis. Probiotic use during the infection is of limited value. Live culture yogurt or dairy is unsuitable as this may aggravate the diarrhea (secondary lactose intolerance). Speak to a pharmacist about probiotics in capsule form.

Last updated on June 9, 2019.

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