The stomach bug, medically referred to as gastroenteritis, is an infection of the stomach and intestines that leads to diarrhoea, vomiting, abdominal cramps, fever, and general weakness. When people ask what causes the stomach bug, the answer is usually related to viral, bacterial, or parasitic infections that disrupt the digestive system.
Contaminated food and water, poor hygiene, and close contact with infected individuals are the most common ways it spreads. While most cases resolve on their own, understanding the causes helps patients manage symptoms and take preventive steps.
What are the viral causes of the stomach bug?
- Norovirus: About 50% of stomach bugs in Australia are caused by this virus. It spreads rapidly in places where people gather closely, such as schools and aged care facilities. Even a very small amount of the virus can trigger infection, which makes outbreaks hard to control. Symptoms tend to appear quickly, within 12 to 48 hours of exposure.
- Rotavirus: Rotavirus is a leading cause of gastroenteritis in infants and young children. Before vaccines became available, it was a major reason for hospital admissions due to severe dehydration. Children infected with rotavirus often experience frequent watery diarrhoea, vomiting, and fever, which can be dangerous without proper hydration.
- Adenovirus: Adenovirus can infect the digestive tract as well as the respiratory system. In children, adenovirus can cause prolonged episodes of diarrhoea that last longer than viral gastroenteritis caused by norovirus or rotavirus. Although this is a less common virus, it can still spread through close contact or contaminated food and water.
- Astrovirus: Astrovirus is another viral cause of the stomach bug, particularly affecting children, older adults, and people with weak immune systems. While symptoms are often milder than those of norovirus or rotavirus, they can still lead to diarrhoea, nausea, and dehydration if not properly managed. It spreads through contaminated food or water.
What are the bacterial causes of the stomach bug?
Salmonella
Salmonella is often associated with raw or undercooked eggs, poultry, and contaminated produce. Infection can result in diarrhoea, abdominal cramps, fever, and, in some cases, blood in the stool. Although symptoms usually resolve without treatment, severe cases can require medical care, especially in young children and older adults.
Escherichia coli (E. coli)
Certain strains of E. coli, such as E. coli O157:H7, can release toxins that damage the lining of the intestines. This leads to severe abdominal pain, watery or bloody diarrhoea, and sometimes kidney complications. Contaminated water and undercooked meat are common sources of infection. Even small amounts can cause infection.
Campylobacter
Campylobacter is a common bacterial cause of gastroenteritis and is often linked to undercooked chicken or cross-contaminated kitchen surfaces. Symptoms usually begin within two to five days of exposure and can include diarrhoea, fever, nausea, and cramps. In rare cases, it can trigger long-term complications such as arthritis.
Cholera
Cholera, though rare in most developed countries, is caused by Vibrio cholerae bacteria. It spreads through contaminated water and produces large volumes of watery diarrhoea, leading to dehydration within a matter of hours. Cholera outbreaks are more common in areas with poor sanitation and limited access to clean drinking water.
Parasitic and uncommon causes
- Giardia: Giardia is a parasite that causes prolonged diarrhoea, bloating, and fatigue. It spreads through contaminated water supplies, making it a risk for people who hike, camp, or travel to regions with unsafe water. The infection can last for weeks without treatment.
- Cryptosporidium: Cryptosporidium is another parasite linked to contaminated water and swimming pools. It can survive standard chlorination, making outbreaks difficult to control. The infection often causes watery diarrhoea that can persist for up to two weeks or longer.
- Chemical toxins: Certain toxins produced by bacteria in spoiled food, such as those from Staphylococcus aureus, can trigger sudden vomiting and diarrhoea. These episodes are sometimes confused with stomach bugs, although they are technically caused by food poisoning.
- Medication reactions: Some medications, particularly antibiotics and certain types of cancer treatments, can irritate the digestive system and cause diarrhoea, nausea, and abdominal cramps. While not infections, these kinds of reactions can mimic the symptoms of the stomach bug.
How does the stomach bug spread?
Person-to-person contact
Close contact with someone who is infected is one of the most common ways the stomach bug spreads. Handshakes or sharing food with an infected person can all transmit viruses and bacteria. It can spread before symptoms appear.
Contaminated food
Improperly cooked or poorly stored food can carry harmful pathogens. Outbreaks of bacterial gastroenteritis are often traced back to contaminated meals prepared in large batches, such as at restaurants or catered events.
Unsafe water
Drinking untreated or contaminated water is a significant source of infection, particularly in regions with poor sanitation practices. Even recreational water from lakes or swimming pools can potentially spread the stomach bug.
Shared surfaces
Viruses like norovirus can survive for days on hard surfaces. Touching contaminated objects such as door handles, phones, or keyboards, then touching your mouth, can lead to infection. Regular handwashing helps reduce this risk.
Risk factors for developing the stomach bug
Age-related vulnerability
Infants, young children, and older adults are more likely to experience severe stomach bugs because their immune systems cannot respond as effectively.
Pre-existing health conditions
People with chronic illnesses, including diabetes and kidney disease, often have weaker immune defences, making them more likely to experience complications.
Travel exposure
Visiting countries where clean water is limited increases the risk of contracting gastroenteritis. Travellers’ diarrhoea is a common example of this risk.
Weakened immunity
Those undergoing chemotherapy or taking immunosuppressive medications are potentially more vulnerable to longer and more severe cases of gastroenteritis.
FAQs
Symptoms often appear within a day of infection and usually resolve within a week, though in some cases they may persist for longer.
Both share similar symptoms, but timing differs. Stomach bugs usually start one to two days after infection, while food poisoning appears within hours of eating contaminated food.
Symptoms typically appear 1 to 2 days after exposure. Most people feel better within a couple of days, but they can remain contagious for several days, and the virus may stay in the stool for up to two weeks.
Yoghurt with probiotics can support digestion and help with diarrhoea, but rich dairy foods may not ease nausea during a stomach bug.
Let an expert help you understand what causes the stomach bug
Understanding what causes the stomach bug provides valuable insight into how this common illness spreads and why it affects people differently. Viruses like norovirus are the most frequent culprits, but bacteria, parasites, contaminated food, and unsafe water can also play a role. Although most cases improve with rest and fluids, vulnerable groups such as children and older adults should take extra care.
Recognising what causes the stomach bug and risks empowers patients to prevent infection and recover safely when illness occurs. Get in touch with a medical professional today to learn how to treat the stomach bug and more.
MEET DR. SUHIRDAN
Interventional Gastroenterologist and Hepatologist
Bsc (Med) | M.B.B.S. (Hons) | FRACP
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