The flu is characterized by a fever, aches, sore throat, and nausea. Seasonal flu epidemics are caused by the influenza viruses A and B. There are several subtypes of influenza A that also circulate every year. The natural source of influenza A is wild birds, but they can infect a host of other animals, including pigs and humans. The viruses are able to mutate very quickly, shuffling proteins around to avoid the immune system of their host. Two or even three different viruses can recombine with one another in a single host and give rise to new, often more infectious, strains. Every year, scientists predict the strains most likely to be present in the human population based on the subtypes seen the previous year, and this prediction is used to design an appropriate flu vaccine. Most influenza A strains affect very young or very old people, because their immune systems are not able to mount a strong enough immune response.
The term stomach flu is usually used to refer to the condition characterized by the sudden onset of vomiting and diarrhea. It's actually a misnomer — the term flu is related to the influenza virus, which is not at all a cause of this condition. It's more appropriately called gastroenteritis, stomach bug, or winter vomiting disease. Several different microbes can cause this condition, including
Viruses like rotavirus and norovirus, which infect children and adults, respectively: These viruses are infectious, pass from person to person, and can still be passed on days after a person's symptoms have stopped.
Bacteria such as Campylobacter, E. coli, Salmonella, and Shigella: These bacteria usually come from another source, such as undercooked contaminated meat, but they can be spread between people if the people come in contact with the stool or vomit of a sick person.
Gastroenteritis from bacteria or viruses usually lasts 24 to 48 hours and is accompanied by a mild fever, painful bloating, and aches and pains. Dehydration can be a dangerous side effect, so it's important that sick people drink clear fluids and are monitored for worsening of symptoms.
Traveler's diarrhea is a type of gastroenteritis that occurs in people as they travel, specifically to the developing countries of Latin America, Africa, the Middle East, and Asia. It's most often caused by enterotoxigenic Escherichia coli (ETEC for short), but it can also be caused by any of the organisms that cause general gastroenteritis. Exposure is from water or food contaminated with fecal matter, and the symptoms are identical to those for gastroenteritis. Preventive treatments are available. Antibiotics, although effective are not recommended because they're a major cause of antibiotic resistance and have side effects on the body. Pepto-Bismol is effective, but it doesn't work for viruses and can't be used for more than a week or two. Finally, an over-the-counter cholera vaccine, also aimed at ETEC, is available that has an effectiveness of between 25 percent and 50 percent. In the end, you may be better off simply paying attention to the foods and beverages you consume to avoid undercooked meats, unpeeled fruit, and tap water.
