Advice about eating out, particularly while travelling, tends towards blanket avoidance of street food. That's poor advice — it excludes some of the best food available and it doesn't actually track the risk well.
Risk is determined by specific factors, most of which you can observe.
What actually causes illness
The main mechanisms are worth knowing because they explain the guidance.
Time and temperature. Bacteria multiply rapidly in the range roughly between 5 and 60 degrees. Food held in that range for extended periods accumulates bacterial load. This is the single most important factor.
Cross-contamination. Raw ingredients contacting cooked food, or the same surfaces and utensils used for both.
Contaminated water. Used for washing, for ice, or as an ingredient.
Handling. Contamination from hands, particularly with food that isn't cooked afterwards.
Why turnover matters more than appearance
The most useful single heuristic: busy is safer than clean-looking.
A stall with constant queues sells food fast, which means it hasn't been sitting. It also means ingredients are turned over frequently rather than kept from yesterday.
A pristine but empty establishment may be holding food for hours.
This is why the standard advice to eat where locals eat is sound, though not because locals have better immunity — it's because volume correlates with freshness.
Cooked to order beats held
The second key factor.
Food cooked in front of you, at high heat, and served immediately, has just been through a process that kills most pathogens and hasn't had time to be recontaminated.
Food sitting in a tray, at ambient temperature, being served from over hours, is the higher-risk category regardless of the setting.
Which means a freshly fried item from a cart is frequently safer than a buffet in a hotel.
The specific higher-risk categories
Not to avoid absolutely, and worth applying more scrutiny to.
Anything with uncooked water. Ice, drinks made with tap water, salads and fruit washed and not peeled.
Raw and lightly cooked items. Salads, chutneys made with raw ingredients, anything not heated.
Dairy-based items held at ambient temperature. Particularly sweets and drinks containing milk or cream.
Reheated rice. The specific issue with rice and its spore-forming bacteria applies here too.
Shellfish, which concentrate contaminants from their environment.
Sauces and condiments left out, particularly those containing egg or dairy.
What to observe
Practical things you can see in thirty seconds.
Is food being cooked to order or served from a holding tray?
Is there a queue?
Is raw food stored separately from cooked?
Is there running water for handwashing, or a visible arrangement for it?
Are the same hands handling money and food? A stall where one person cooks and another takes payment is a good sign.
Is the oil in the fryer dark and viscous, suggesting it's been used for a long time?
Are cut fruits sitting exposed, or being cut when ordered?
Travel-specific considerations
An honest point that gets muddled. Travellers frequently get ill in places where residents don't, and the main reason isn't standards — it's exposure.
People develop some resistance to the specific organisms common in their own environment. Arriving somewhere new means encountering strains you haven't met.
Which means a stall serving hundreds of local customers without incident can still make a visitor ill. That's not the vendor's failure and it does mean caution is reasonable early in a trip.
Practical approach: start conservatively, eat cooked hot food for the first few days, and expand as you adjust.
Bottled or treated water, and avoiding ice unless you're confident about its source, remains sensible advice in areas where water quality is variable.
What to do if it happens
Most food-borne illness resolves in a day or two with rest and fluid replacement.
Oral rehydration solution is considerably more effective than plain water for replacing fluid and electrolytes, and is cheap and widely available.
Anti-motility medications can help with symptoms and are generally not recommended if there's fever or blood in stool, since they can prolong some infections.
Seek medical attention for high fever, blood in stool, signs of significant dehydration, symptoms persisting beyond a few days, or if you're in a group where dehydration is more dangerous — young children, older people, anyone with an existing condition.
General information only. If you have concerns about symptoms, please seek medical advice.
The home kitchen comparison
Worth some perspective. A substantial proportion of food-borne illness originates in domestic kitchens rather than in commercial ones, which is not the impression most advice gives.
The mechanisms are the same: food left out too long, inadequate refrigeration, cross-contamination between raw and cooked, and reheating that does not reach a sufficient temperature throughout.
Which suggests some domestic habits worth examining. Fridge temperature, which most people have never checked and which is frequently higher than it should be. Separate boards or thorough washing between raw meat and anything eaten uncooked. Cooling cooked food quickly rather than leaving it on the hob to cool slowly.
None of that argues against being careful when eating out. It does suggest that scrutiny applied to a street vendor might reasonably also be applied at home.