A co-worker was telling me statistics say that on average a person only gets the flu once in their life. The rest of the time it's just some stomach bug or something. I don't know how true that is, it could be bunk.

Most people who get influenza will recover in one to two weeks, but others will develop life-threatening complications (such as pneumonia). According to the World Health Organization: "Every winter, tens of millions of people get the flu. Most are home, sick and miserable, for about a week.

As influenza is caused by a variety of species and strains of viruses, in any given year some strains can die out while others create epidemics while yet another strain can cause a pandemic. Typically, in a year's normal two flu seasons (one per hemisphere) there are between three and five million cases of severe illness and up to 500,000 deaths worldwide, which by some definitions is a yearly influenza epidemic.[68] Although the incidence of influenza can vary widely between years, approximately 36,000 deaths and more than 200,000 hospitalizations are directly associated with influenza every year in America.[69][70][71] Every ten to twenty years a pandemic occurs, which infects a large proportion of the world's population, and can kill tens of millions of people (see history section).

It is possible to get vaccinated and still get influenza. The vaccine is reformulated each season for a few specific flu strains, but cannot possibly include all the strains actively infecting people in the world for that season. It takes about six months for the manufacturers to formulate and produce the millions of doses required to deal with the seasonal epidemics; occasionally, a new or overlooked strain becomes prominent during that time and infects people although they have been vaccinated (as by the H3N2 Fujian flu in the 2003–2004 flu season).[76] It is also possible to get infected just before vaccination and get sick with the very strain that the vaccine is supposed to prevent, as the vaccine takes about two weeks to become effective.[55]

Most people who get influenza will recover in a few days to less than 2 weeks, but some people will develop life-threatening complications (such as pneumonia) as a result of the flu. Millions of people in the United States — about 5% to 20% of U.S. residents — will get influenza each year. An average of about 36,000 people per year in the United States die from influenza, and more than 200,000 have to be admitted to the hospital as a result of influenza.

Your respiratory illness might be the flu if you have sudden onset of body aches, high fever, and respiratory symptoms, and your illness occurs during the usual flu season in the Northern Hemisphere. However, during this time, other respiratory illnesses can cause similar symptoms to the flu. In addition, influenza can also occur outside of the typical flu season. It is impossible to tell for sure if you have the flu based on symptoms alone. Doctors can perform tests to see if you have the flu if you are in the first few days of your illness.

Many people use the term "stomach flu" to describe illnesses with nausea, vomiting or diarrhea. These symptoms can be caused by many different viruses, bacteria or even parasites. While vomiting, diarrhea, and being nauseous or "sick to your stomach" can sometimes be related to the flu – particularly in children – these problems are rarely the main symptoms of influenza. The flu is a respiratory disease and not a stomach or intestinal disease.