SINGAPORE - Two separate mass food poisoning cases hit the Republic in July, with 165 Singapore Civil Defence Force (SCDF) personnel and 130 employees of Chinese tech company ByteDance falling ill in the space of about a week.

       Between July 23 and 24, SCDF personnel at the Civil Defence Academy reported sick due to food poisoning. They suffered mild gastroenteritis, which included symptoms of abdominal discomfort and diarrhoea.

       On July 30, the SCDF received several calls from ByteDance employees who were vomiting and in severe abdominal pain. Two catering businesses were suspended for their links to the case, which left 130 employees severely ill and several vomiting into dustbins when they could not make it to the toilet in time. One caterer has since had its suspension lifted.

       The Straits Times breaks down the science behind how cooked food goes bad, and why we get sick after eating it.

       Food poisoning, or gastroenteritis, is the body’s reaction to toxins that a person consumes through food.

       Symptoms include vomiting, diarrhoea, nausea and abdominal pain. These are the body’s ways of purging the toxins.

       Food can be contaminated with a toxic organism, such as bacteria or a virus. Sometimes, the toxic by-products of these organisms can cause food poisoning as well.

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       Viruses are microscopic parasites that cannot exist on their own.

       This means that they need a living host – such as a human cell – in order to survive and reproduce.

       A body experiencing a viral attack is similar to a cruise ship getting ambushed by pirates, said Professor William Chen, director of Nanyang Technological University’s (NTU) Food Science and Technology Programme and NTU’s Future Ready Food Safety Hub.

       The virus hijacks and damages the body’s intestinal cells, like how pirates attack the cruise ship’s passengers and deplete the ship’s resources until a new target is identified, explained Prof Chen.

       The virus will then move on to another body.

       Viruses that can cause food poisoning include the norovirus and rotavirus, which can be found in raw or undercooked shellfish, and the vomit and faecal particles of an infected person.

       When food contaminated with norovirus or rotavirus is consumed, the viruses invade the host cell and cause cell death, said Dr Aaron Poh, consultant colorectal surgeon and medical director of Alpine Surgical Practice.

       Intestinal cells are unable to perform their usual function of absorbing water, leading to diarrhoea.

       Not all bacteria cause illnesses in humans.

       Some bacteria can be beneficial, including those found in the human gut microbiome, as well as those used in the fermentation of wine, bread and kimchi.

       However, “bad” bacteria – such as salmonella, Escherichia coli (E. coli), listeria and Bacillus cereus (B. cereus) – produce toxins that may cause diarrhoea or vomiting.

       If viruses are like pirates, then bacteria are like passengers on a cruise, Prof Chen said.

       Good passengers cause no harm, while bad passengers wreak havoc by littering or vandalising the cruise ship, like how harmful bacteria create toxins that sicken the body.

       Unlike viruses, for which doctors may not have specific treatments, bacteria can be killed by antibiotics, said Dr Poh.

       Depending on the bacteria or virus causing the gastroenteritis, symptoms can manifest in diverse ways.

       Bacterial food poisoning tends to have quicker onset. A person might feel symptoms within minutes to hours of eating the contaminated food, but this should generally last only up to two or three days.

       Food poisoning by viruses tends to take longer to manifest and lasts for up to a week or 10 days.

       The symptoms of both can be quite similar: diarrhoea, vomiting and abdominal pain.

       A low-grade fever can also occur, but if the fever is high, it could either be a more severe case of food poisoning, or it could signal something else entirely, said Dr Poh.

       Dr Benjamin Yip, consultant gastroenterologist and medical director of Alpha Digestive and Liver Centre, also noted that food poisoning caused by bacteria tends to be more severe, causing bloody diarrhoea and higher fever.

       Anyone can get food poisoning, but the very young and very old will be more severely affected as their immune systems tend to be weaker, said Dr Poh.

       As for whether some people are naturally more susceptible to food poisoning, he said: “I don’t think, per se, there is someone who is more vulnerable to food poisoning. If there is, it’s because they may already have a pre-existing disease.”

       You might not be able to tell by looking at the food, or even smelling it alone.

       This is because bacteria can affect your food in two ways: by causing food spoilage or food poisoning.

       Food spoilage is when there is a change in the food quality, resulting in food poisoning when the spoilt food is consumed.

       For example, milk that has been left out in the open turns sour because bacteria in the milk produces acids and waste products. Its sour smell signals that it has gone bad. If consumed, the spoilt milk could make the person sick.

       But foods contaminated with bad bacteria might not look or smell any different.

       The nasi lemak sitting on your table might look or even taste fine, but because the food vendor did not wash his hands, it might have been contaminated with harmful bacteria.

       Harmful bacteria produce toxins that stay on the food. While the toxin-producing bacteria can be killed by heating up the food, the toxins they leave tend to be more heat-resistant and cannot be eradicated as easily.

       As with most illnesses, early detection is key. If you think you might have eaten food that has gone bad, it is important to look out for symptoms and seek medical attention if necessary.

       Information such as when the symptoms began appearing, what food you might have eaten, and whether another person you know who ate the same food is also displaying similar symptoms are important to note, so doctors will know how to treat you, said Prof Chen.

       For milder cases, patients should remember to stay hydrated, as vomiting and diarrhoea result in major loss of fluids. Dr Yip recommends buying over-the-counter oral rehydration salts or making the solution at home with glucose and table salt, in order to keep the body healthy with nutrients essential for bodily functions.

       “The key thing that always differentiates (between more and less severe cases) is whether (a patient is) able to retain fluids,” said Dr Poh.

       More severe cases involve such frequent vomiting that the body cannot replenish all the fluid it is losing.

       In these cases, patients are at greater risk of dehydration and would be recommended to be hospitalised for intravenous medication. They will be injected with fluids to replenish fluid and nutrients lost, and medicine to reduce nausea to tide them over the acute phase.

       As bacteria thrive in warmer temperatures, Singapore’s warm climate favours the growth of food-borne pathogens and puts us at greater risk of experiencing food poisoning.

       As we cannot see with the naked eye when we are exposed to pathogenic micro-organisms or toxins through food, “prevention through the application of basic food hygiene practices is our most powerful tool”, said Prof Chen.

       Food must be cooked well and refrigerated at cool temperatures instead of being left in the open.

       Food handlers must be careful to prevent cross-contamination, including washing their hands before and after food preparation, and ensuring that all utensils and cutlery are thoroughly sanitised.