Not witchcraft or bad luck: Expert explains why food poisoning hits people differently

Food poisoning is a common concern across Kenya, yet many misconceptions surround what actually causes people to fall ill after eating.

From street food vendors to home-cooked meals, Kenyans frequently encounter situations where some people become severely sick after a shared meal, while others remain perfectly fine. Is it deliberate poisoning? Bad luck? Or is there a scientific explanation behind these seemingly random outcomes?

To shed light on these concerns, Health Nation spoke with Beatrice Mosii, a microbiologist with extensive experience in food safety and laboratory testing in Kenya.

From a microbiological standpoint, what actually causes most cases of what people call "food poisoning"?

Most cases are not deliberate poisoning but are caused by bacterial contamination that occurs accidentally during food production, preparation, or storage. The main contributing factors include uneven distribution of bacteria throughout the food, differences in individual immunity levels among those eating, varying portion sizes consumed by different people, the timing of when each person eats from the same batch, differences in stomach acidity between individuals, and pre-existing gut conditions or immunity that some individuals may have developed from previous exposures.

Why don't all people who eat the same meal get sick? If the food is contaminated, shouldn't everyone who eats it become ill? What's the scientific explanation for these different outcomes?

Several factors work together to create different outcomes for different people, and understanding this helps dispel the myth that unaffected diners somehow "got lucky." Some individuals have prior exposure to the same microorganism and have already developed antibodies, so they show no symptoms even after consuming contaminated food—their immune system recognises and neutralises the pathogen before it causes problems.

Portion size matters significantly because someone who eats a larger amount ingests more bacteria, resulting in a higher infectious dose that overwhelms their immune defences. The bacteria are not uniformly distributed throughout the food, so one person may get a heavily contaminated piece while others get portions with almost no contamination.

The specific foods each person chooses from a shared meal also matter greatly; for example, one person takes rice while another skips it entirely, and if the rice happens to be the contaminated item, only those who ate it are at risk. Finally, differences in stomach acidity levels and overall immune status play crucial roles in determining who becomes ill, as stomach acid is the body's first line of defence against ingested pathogens.

Can you clarify the difference between food contaminated with bacteria and food deliberately poisoned with toxic substances?

Bacterial contamination usually happens accidentally because of poor hygiene practices, improper cooking techniques, contaminated water sources, or inadequate storage conditions that allow bacteria to multiply to dangerous levels. Deliberate poisoning, on the other hand, involves intentionally adding toxic chemicals or administering high doses of pathogens with malicious intent—this is a criminal act rather than a food safety failure.

In everyday cases throughout Kenya, almost all incidents referred to as "food poisoning" are actually accidental bacterial contamination rather than deliberate acts of poisoning. The confusion arises because both can make people sick, but the mechanisms, prevention strategies, and legal implications are completely different. True chemical poisoning would typically affect everyone who consumed the food equally and produce symptoms rapidly, whereas bacterial contamination shows the variable patterns we discussed earlier.

How do timing and temperature affect food poisoning outcomes?

Different pathogens have different incubation periods, which explains why symptoms don't always appear at the same time or immediately after eating. Some bacteria like Staphylococcus aureus produce toxins that cause symptoms within just a few hours of consumption—these are pre-formed toxins that were already in the food before you ate it.

Others like Salmonella or E. coli need time to colonise your intestines and multiply before producing enough toxins or damage to cause symptoms, which may take anywhere from 12 to 72 hours or even longer before symptoms manifest.

Temperature abuse, particularly leaving food at room temperature for extended periods, allows bacteria to multiply rapidly both before and after cooking, dramatically increasing the risk of illness. Every hour that food sits in the "danger zone" between 5°C and 60°C, bacterial populations can double, turning a relatively safe amount of contamination into a dangerous dose.

Which foodborne pathogens are most common in Kenya, and what specific foods or food practices are they most commonly associated with?

The most frequently encountered pathogens include E. coli and other coliforms, which are found in raw or undercooked meat, unpasteurised milk, and contaminated water used for washing produce or food preparation surfaces.

Salmonella is commonly associated with poultry, meat, and eggs, particularly when these are not cooked to proper internal temperatures or when raw products cross-contaminate ready-to-eat foods. Shigella contamination typically comes from contaminated water, salads, and foods handled by infected persons who haven't practised proper hygiene—this is often called the "faecal-oral route" of transmission.

The Enterobacteriaceae family, in general, is widely present in meat and milk sold at local markets, often at levels that exceed safe limits. Street-vended milk and meat sold in open markets often test positive for these pathogens during laboratory analysis, with some samples showing bacterial counts that are orders of magnitude above acceptable standards. The informal nature of these markets, with limited refrigeration and hygiene facilities, creates ideal conditions for bacterial growth and transmission.

You've mentioned contaminated water several times. Why is water quality so critically important in food safety? Can't proper cooking kill any bacteria that water might introduce?

Contaminated water is one of the biggest sources of foodborne bacteria in both homes and small eateries across Kenya, yet it's often the most overlooked factor in food safety. People may cook their food properly and follow good practices during preparation, but if they use contaminated water to wash vegetables, clean utensils, rinse milk containers, or even wash their hands, they effectively re-contaminate everything they've worked to keep safe.

Cooking does kill bacteria, but water contamination often happens after cooking, when people wash cooked food, dilute milk, make drinks, rinse plates that will hold cooked food, or use contaminated hands to serve food. Most households and small businesses never test their water quality, so they remain unaware of this hidden source of contamination that undermines all their food safety efforts. The water may look clean and clear but still contain dangerous levels of bacteria, especially in areas relying on wells, boreholes, or municipal supplies with intermittent treatment.

Based on your laboratory experience and observations, what are the most common food-handling mistakes you see in Kenyan households that lead to contamination?

The most frequent mistakes include using untreated or contaminated water for drinking, cooking, and cleaning purposes without any awareness of the risk this poses. Many people don't wash their hands properly before handling food, or they skip handwashing entirely, especially after using the toilet or handling raw meat. Inadequate cooking, especially of meat and poultry, leaves dangerous bacteria alive in the food—people often judge doneness by appearance rather than using a thermometer or ensuring proper cooking times.

Cross-contamination occurs when people use the same knife or cutting board for raw meat and then for ready-to-eat foods without proper washing in between, effectively transferring bacteria from raw to cooked products. Finally, leaving cooked food at room temperature for too long allows any surviving bacteria to multiply to dangerous levels before the food is consumed—this is especially common when food is prepared hours in advance for events or when leftovers are not refrigerated promptly. These mistakes are often cultural or convenience-based rather than knowledge gaps, making them harder to address through education alone.

How can ordinary people test their water at home? Are there simple test kits available, or do they need professional laboratory services?

You cannot do proper microbiological testing at home with any reliability, despite what some commercial test kits might claim. You need to take a water sample to an accredited laboratory such as Aqualytic Laboratories or other certified labs operating in Kenya.

These laboratories can test for total coliforms, E. coli specifically, total viable bacterial count, Pseudomonas, and other important indicators of water safety that require specialized equipment, trained technicians, and proper quality control procedures.

Home testing kits simply cannot provide the accuracy and comprehensive analysis needed to make informed decisions about water safety, as they may give false positives or false negatives that create a false sense of security or unnecessary panic.

Are foodborne illnesses underreported in Kenya? What do the official statistics miss, and why does this underreporting matter for public health?

Yes, they are severely underreported, and the true burden of foodborne disease in Kenya is likely 10 to 100 times higher than official figures suggest. Most people who get sick after eating a meal just go to the local chemist and buy antibiotics like metronidazole or flagyl without seeing a doctor, or they simply decide to avoid that particular food or vendor in the future and consider it a personal lesson learned.

Few people report their illness to public health authorities, partly because they don't know they should, partly because the process seems complicated, and partly because they don't believe anything will be done about it. This underreporting makes it difficult to identify problem areas and implement effective interventions—we can't fix problems we can't see or measure.

It also means dangerous food vendors can continue operating for years, making hundreds of people sick, without ever facing consequences or being identified as a public health threat. Better reporting would allow health authorities to identify patterns, trace outbreaks back to their sources, and prevent future cases.

Any final advice for people concerned about food safety? What are the highest-impact actions that ordinary Kenyans can take today to protect themselves and their families?

Start with your water, get it tested at a certified laboratory to establish a baseline for safety, and then treat it appropriately based on the results, whether that means boiling, using proper filtration, or chlorination.

Cook all food thoroughly, especially meat and poultry, and use a food thermometer if possible rather than relying on appearance alone—internal temperatures should reach at least 75°C for most foods. Practise good hygiene, including proper handwashing with soap before food preparation, after using the toilet, and after handling raw meat or eggs.

Consider periodic testing of high-risk foods like meat, milk, and ready-to-eat salads, particularly if you operate a food business or serve vulnerable populations like children, elderly people, or those with compromised immune systems.

Education and routine testing are the biggest gaps we currently have in Kenya's food safety system, so taking personal responsibility for these areas can significantly reduce your risk of foodborne illness. Remember that food safety is not about eliminating all bacteria that's impossible but about keeping bacterial levels low enough that your immune system can handle any exposure without becoming ill.

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