*The flu (or common flu) is a viral infection that is spread from person to person in secretions of the nose and lungs, for example when sneezing. Medically, it is referred to as influenza.

Flu is a respiratory infection that is an infection that develops primarily in the lungs. Respiratory infections caused by other viruses often are called flu, but this is incorrect. Influenza usually causes higher fever, more malaise, and severe body aches than other respiratory infections.

Although other viruses may cause these symptoms, they do so less commonly than the flu. Influenza viruses are divided into three types, designated A, B, and C. Influenza types A and B are responsible for epidemics of respiratory illness that occur almost every winter. Influenza type C usually causes either a very mild respiratory illness or no symptoms at all; it does not cause epidemics and does not have the severe “public-health” impact of influenza types A and B.

Type A viruses are divided into subtypes and are named based on differences in two viral surface proteins called hemagglutinin (H) and neuraminidase (N). There are 16 known H subtypes and nine known N subtypes. The flu is a very common illness. Every year in the United States, on average 5%-20% of the population gets the flu, more than 200,000 people are hospitalized from flu complications, and about 36,000 people die from the flu or its complications.

Each year, the influenza virus can change slightly, making the vaccine used in previous years ineffective. A new vaccine must be prepared every year, which will be effective against the expected type of influenza virus. Every year, federal health agency officials try to guess which three flu strains are most likely to be prevalent in the U.S. the following year to determine which strains will be included in next year’s flu vaccine. If they guess right, the vaccine is thought to be 70 to 80 percent effective in temporarily preventing the flu of the season in healthy persons less than 65 years old (the effective rate drops to 30 to 40% in those over 65 years old but the vaccine is thought to be 50 to 60% effective in preventing hospitalization and pneumonia and 80% effective in preventing death from the flu in the over 65 age group).

However, sometimes health officials do not correctly predict which flu strains will be most prevalent and the vaccine’s effectiveness is much lower for that year. The trick is to be able to predict which influenza viruses are going to cause infection and to prepare a vaccine against those viruses. Usually, scientists can predict accurately which types of influenza virus will cause infections and prepare an appropriate vaccine. The viruses that are used to prepare flu vaccine are grown in hen’s eggs.

Traditionally, we’ve given influenza vaccines during a tight period of time in October and November of each year, mostly because we were trying to focus on high-risk groups. The most frequent side effects are pain, redness, and swelling at the injection site (10-64 %) lasting 1-2 days, and systemic side effects such as headache, fever, and malaise in about 5 % of persons who are vaccinated.

The “swine flu” pandemic of 2009 is caused by a “new” influenza A virus designated H1N1 based upon its protein types. This virus was originally referred to as “swine flu” because many of the genes in this new virus were very similar to influenza viruses that normally occur in pigs in North America. However, this new virus is actually quite different from the typical swine flu viruses found in pigs.

The H1N1 virus first caused illness in Mexico and the United States in March and April 2009. H1N1 flu is spread from person to person, unlike typical swine flu, although it is not clear how easily the virus is able to spread among people. The government and vaccine manufacturers are conducting clinical trials, right now, to determine whether the vaccine is effective and how large a dose is needed.

Initial results from these “trials” are expected in early October. Some experts and advocates (myself included) have expressed concern that the vaccine may end up being administered to pregnant women and children before full test results are in. But government officials believe the new vaccine is safe because it resembles seasonal flu vaccines, which normally don’t undergo trials.

Here are some basic facts about how the flu vaccine works: * It takes about two weeks for the flu vaccine to build protection in your body.

* The flu vaccine does not cause the flu.

* No vaccine protects 100% from disease. There is a chance you may become ill with the flu even after you get the vaccine. If you happen to get influenza after the vaccine the symptoms should be much milder.

* The vaccine does not protect against colds or other illnesses that have symptoms like influenza. Although influenza is primarily spread by droplet transmission, the virus can also live on objects such as doorknobs, telephone receivers, utensils and food trays, beds and medical equipment for possibly up to one day. Some people infected with influenza may not develop symptoms at all, but may be infectious to others. For infected persons who do develop symptoms, they can be contagious the day before they get symptoms. So nurses can transmit the virus even before they realize they are infected. The period of greatest contagion is during the first three days of illness, and can last for five to seven days in otherwise healthy adults.

Remember, I’m not a doctor, I just sound like one. Take good care of yourself and live the best life possible! This information is not intended to take the place of proper medical advice from a healthcare professional or medical doctor.

Glenn Ellis, author of Which Doctor?, is a health columnist and radio commentator who lectures around the country on relevant health issues. For good health information, visit: www.glennellis.com