Gastro in Adults: The 5 Stages and When to See a Doctor
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"Gastro" is shorthand for gastroenteritis — inflammation of the stomach and intestines that causes vomiting, diarrhoea, cramping, and sometimes fever. In adults, the most common causes are:
- Viruses — norovirus and rotavirus are the big two. Viral gastro is the most common cause of acute gastro in healthy adults, spreads extraordinarily easily, and usually resolves on its own.
- Bacteria — Salmonella, Campylobacter, E. coli, and Shigella. Often tied to contaminated food or water. Symptoms can be more severe and may last longer.
- Parasites — Giardia is the most common in Australia, especially in people who've been camping, drinking untreated water, or travelling.
- Food poisoning — a broad term that overlaps with the above.
Most gastro in otherwise healthy adults is viral, self-limiting, and uncomfortable but not dangerous. The risk is dehydration, not the infection itself — which is why understanding the timeline matters.
Gastro doesn't always follow a neat progression, but there's a common clinical pattern worth knowing so you can match what's happening to what's normal.
Stage 1 — Incubation (6 hours to 3 days). You've been exposed but don't know it. No symptoms yet. You may already be contagious toward the end of this stage. For a deeper look at how long this takes by cause, see Gastro Incubation Period.
Stage 2 — Onset (the first 24 hours). Symptoms hit — often fast. Nausea, vomiting, cramping, and the first bouts of diarrhoea. Many people also feel feverish, achy, and exhausted. This is usually the most acute stage.
Stage 3 — Peak illness (24–72 hours). The worst of the vomiting and diarrhoea. Dehydration risk is highest here, because you're losing fluid faster than you can replace it. Most healthy adults stay on the couch or in bed.
Stage 4 — Resolution (days 3–5). Vomiting typically stops first, diarrhoea eases. Energy is still low. Appetite starts returning but tolerance for normal food is limited. You'll often feel flat for a few more days.
Stage 5 — Recovery (days 5–10, sometimes longer). Bowel habits return toward normal. Energy rebuilds. Some people remain contagious for up to two weeks after symptoms resolve, particularly with norovirus — important if you work with food, kids, or elderly relatives.
Bacterial and parasitic gastro can extend these windows — sometimes by several days, sometimes by a week or more. That's one of the signals it might not be a routine viral bug.
Normal features of uncomplicated adult gastro:
- Vomiting that eases within 24–48 hours
- Watery diarrhoea that gradually settles over 2–5 days
- Mild fever (under 38.5°C) for a day or two
- Cramping and bloating through the acute phase
- Fatigue that lingers a few days after other symptoms stop
Things that are not routine and need medical assessment (covered in detail further down):
- Blood in stool or vomit
- High fever (over 39°C) or fever lasting more than 48 hours
- Severe abdominal pain (different from cramping)
- Symptoms lasting more than 5–7 days with no improvement
- Signs of significant dehydration
The five-stage pattern is a rough map, not a diagnosis. If your course is clearly off-pattern, that's useful information — it's one of the things a GP will want to hear about.
For most adults with viral gastro, home care is the mainstay. The priority is fluid and electrolyte replacement, not "stopping" the illness.
Hydration. Small, frequent sips beat big glasses. Water is fine, but once you've lost significant fluid, plain water alone doesn't replace salt and sugar. Oral rehydration solutions (ORS) — available from any pharmacy — are the gold standard. Sports drinks are a distant second (usually too much sugar, not enough sodium) but acceptable if ORS isn't available.
Food. Trust appetite. When vomiting eases, start small — dry toast, plain crackers, rice, bananas. The old "BRAT" diet (bananas, rice, applesauce, toast) has limited evidence behind it but is gentle and unlikely to hurt. Reintroduce normal food as tolerated over 2–3 days.
Avoid while symptomatic. Alcohol, caffeine, heavily spiced food, high-fat food, and — for some people — dairy. Most adults can handle dairy again within a few days; a smaller group develops temporary lactose intolerance lasting a couple of weeks.
Rest. Time off work is legitimate. Most employers accept a medical certificate — for how telehealth can issue one, see how to get an online medical certificate in Australia.
Hygiene. Wash hands thoroughly after every bathroom visit, before preparing any food, and after vomiting. Disinfect toilet seats, taps, and door handles. Norovirus in particular survives on surfaces for days.
Most acute gastro doesn't need prescription medication. But there are a few categories worth understanding.
Over-the-counter symptom relief. A non-prescription pain reliever for fever and body aches is reasonable. Pharmacists can advise on over-the-counter anti-nausea or anti-diarrhoea options — some aren't suitable if you have a fever or blood in stool, which is why asking is better than grabbing off the shelf.
Prescription antiemetics. For persistent vomiting, a GP can sometimes prescribe an antiemetic — the specific medication depends on the cause, your other medications, and clinical judgement. Not appropriate for every case.
Antibiotics. Generally not used for routine gastro. Most gastro is viral, and antibiotics don't help viral illness. They can, in some bacterial or parasitic cases (confirmed on stool testing), be appropriate — but this is a decision based on specific pathogens and clinical context, not on symptoms alone.
We don't name specific medications in this post on purpose. Drug choice depends on the cause, your kidney function, your other medications, and — for antibiotics — confirmed pathogen and local resistance patterns. Self-medicating or reusing leftover antibiotics can mask a more serious infection or contribute to resistance.
Need time off work?
Most gastro settles without medical care. Some of it needs attention promptly.
Book a consult if:
- Vomiting or diarrhoea lasts more than 48 hours with no improvement
- You're struggling to keep any fluid down
- You have a fever over 38.5°C that lasts more than 24 hours
- You've been travelling, camping, or drinking untreated water before symptoms started
- You're pregnant, over 70, have a kidney condition, diabetes, or are immunocompromised
- You need a medical certificate for work
Go to an emergency department — or call an ambulance — if:
- You can't keep any fluid down for more than 12 hours
- Signs of severe dehydration: very little urine, dizziness on standing, confusion, sunken eyes, rapid heart rate
- Blood in stool or vomit (fresh red or dark/coffee-ground)
- Severe abdominal pain that's different from cramping, constant, or localised to one area
- High fever above 39°C with rigors or confusion
- Symptoms in a baby or very young child: dehydration sets in much faster than in adults
Gastro in pregnancy, in older adults, and in anyone immunocompromised can escalate faster than in healthy adults. The bar for seeking care is lower.
Abby Health is an online-first Australian clinic. Our GPs are AHPRA-registered. When you book a consult, Abby AI, our medical AI, prepares a clinical brief for your doctor — including any previous gastro episodes, recent travel, relevant medications, and red-flag symptoms — so they're already informed when you connect.
For adult gastro, your Abby GP can take a history, assess hydration and red flags, issue a medical certificate if you need time off work, and recommend the right symptom-relief approach based on your specific picture. If pathology is needed — stool testing for suspected bacterial or parasitic cases — they'll arrange a pathology referral. If you need an in-person examination or urgent care, they'll tell you directly.
For more on how telehealth prescribing works in Australia, see how telehealth prescriptions work in Australia. If you need a medical certificate specifically, see Doctor's Note vs Medical Certificate in Australia. And for incubation timing and contagious windows, see Gastro Incubation Period.
Abby Health consultations are bulk billed for eligible patients with a valid Medicare card.
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