Sanitation and disease

Public health engineers draw this as the F-diagram: faeces reach a new mouth through fluids (drinking water), fields (soil and crops), flies, fingers and food.

The barriers come in two kinds, and the first kind matters more:

1. Primary barrier — separate faeces from the environment. A latrine that people actually use. This blocks all five routes at once, before anything else is contaminated.
2. Secondary barriers — stop what escaped. Handwashing with soap, safe drinking water, covered food, clean utensils.

The practical consequence: a group that treats its water perfectly and defecates in the open is still sick, because fingers, flies and food carry what the water treatment never saw. Sanitation and water safety are one system. See the water guide.

A latrine in the wrong place is a way of injecting faeces into the water supply. These are the Sphere humanitarian standards — written from what worked and failed in camps holding hundreds of thousands of people.

There is no best latrine, only the right one for the water table, the water supply and the people who will use it. The single most common failure across every type is not engineering — it is a latrine nobody wants to use.

The ventilated improved pit is the most important low-cost latrine design of the last fifty years, and the reason it works is not obvious.

Air moves one way. Wind blowing across the top of the vent pipe, and sun heating it, draw air down through the squat hole and up the pipe. The smell leaves above the roof instead of through the room.

Flies are trapped by light. Flies that breed in the pit fly toward the brightest light when they emerge. If the inside of the shelter is dark, the brightest light is the top of the vent pipe — where a fly screen stops them. They die there and fall back into the pit. Flies outside, drawn by the smell at the vent, meet the same screen from the other side.

Every detail follows from those two mechanisms:

• Vent pipe at least 110 mm in diameter, 150 mm better, reaching at least 0.5 m above the highest point of the roof.
• On the sunny side of the shelter, painted dark if it is not already, so the sun drives the updraught.
• Fly screen of stainless steel or aluminium mesh, holes about 1.2 mm. Plastic and ordinary steel mesh rot through within a year or two, and a torn screen is a fly exit.
• The interior dark. A door kept shut, or a spiral entrance with no door at all. A bright room turns the squat hole into the exit.
• No lid on the squat hole — it would stop the airflow the design depends on.

Handwashing with soap cuts diarrhoeal illness by roughly 30–45% in controlled trials, and respiratory infections substantially as well. It is the cheapest health intervention there is — but only at the right moments, and only with soap.

The five moments that matter:

1. After defecating.
2. After cleaning a child's bottom or handling a nappy.
3. Before preparing food.
4. Before eating.
5. Before feeding a child.

How: wet, lather with soap, scrub every surface — backs, between fingers, under nails — for at least 20 seconds, rinse under running water, and air-dry or shake dry. A shared towel puts back what the soap took off. A shared basin of standing water does the same.

Soap matters. Water alone removes far less. Where there is no soap, wood ash is the traditional substitute — the evidence for it is weaker than for soap, and it is still far better than water alone, because most of the effect is the friction and the rinse. Making soap is its own step.

Building a tippy tap — a handwashing station that needs no plumbing and uses about 40 ml of water per wash:

1. Two forked uprights about 1.5 m tall, a crossbar between them.
2. A 3–5 litre container hung from the crossbar by its handle, with a ~2 mm hole punched just below the cap.
3. A string from the container's neck down to a foot pedal stick — press it and the container tips, and a thin stream runs from the hole. Hands never touch the container.
4. Soap threaded on a string, or ash in a covered tin, hung from the crossbar.
5. A small pit of gravel underneath, so the wash water soaks away instead of making mud and a mosquito puddle.

Put it beside the latrine door, where the moment happens. A handwashing station across the camp is one nobody walks to.

Human faeces are a fertiliser and a disease reservoir at the same time. Two things make them safe, and only these two: heat, or time.

The organism that sets the times is not a bacterium — it is the egg of the roundworm Ascaris, which survives drying, cold and many chemicals, and outlasts nearly everything else. A process that kills Ascaris eggs has killed the rest.

Diarrhoea kills by dehydration, not by infection. That single fact is what makes it survivable without a hospital: replace the water and salts faster than they are lost, and most people recover while the infection runs its course.

Oral rehydration solution (WHO home recipe):

• 1 litre of safe water
• 6 level teaspoons of sugar
• ½ level teaspoon of salt

Stir until dissolved. Too much salt is dangerous — if it tastes saltier than tears, remake it. The sugar is not for energy; glucose drives sodium, and water with it, across the gut wall. That mechanism is why plain water does not rehydrate a cholera patient and this does.

How much: after every loose stool, 50–100 ml for a child under 2, 100–200 ml for ages 2–10, and as much as wanted for older children and adults. Give it in small sips; vomiting is not a reason to stop — wait ten minutes and resume, more slowly. Keep breastfeeding. Give children zinc as well if it exists — 20 mg a day for 10–14 days (10 mg under six months) shortens the illness.

With rehydration alone, cholera case fatality falls below 1%. Untreated, severe cholera can kill in hours.

Killing insects and rodents is endless. Removing where they breed is finite.

• House flies breed in faeces and rotting food, and go from egg to adult in 7–10 days in warm weather. They carry pathogens on their feet from a latrine to a plate. Cover faeces, cover food, and bury or burn food waste — a fly population with nothing to breed in collapses within two weeks.
• Mosquitoes need standing water. Aedes, which carries dengue, Zika and yellow fever, breeds in anything that holds water for a week: buckets, tyres, pots, tin cans, bottle caps, leaf axils. Its cycle from egg to adult is about 7–10 days, so emptying and scrubbing every water container once a week breaks it. Cover water stores; screen rainwater tanks. Anopheles, which carries malaria, prefers clean, still, sunlit pools — drain or fill puddles, and sleep under a net.
• Rats follow food and shelter. Store food in sealed metal, ceramic or thick plastic containers, raise stores off the ground, keep waste in covered pits, and clear the rubbish and long grass beside dwellings. Rat urine spreads leptospirosis into water and wet soil — do not wade barefoot in floodwater near settlements.