I've been asked to run an update on my influenza column that I publish each year. We've been starting to see cases of influenza in central Indiana. According to the CDC, Indiana is listed as having "regional" prevalence of influenza, one step shy of being classified as "widespread."

Most people us the term "flu" in a very generic sense, meaning anything from cold symptoms to having a case of vomiting and diarrhea. The "flu" in this column refers to respiratory influenza.

The majority of influenza infections are caused by two particular types of influenza viruses, Type A and Type B.  Type B typically does not cause severe disease whereas Type A can be fatal, particularly in the young, elderly and in those who have compromised immune systems.

Type A virus can be broken down further into different subtypes or "serotypes" based on the proteins on the surface of the virus. When you read about influenza virus with a name like "H1N1," the "H" and "N" refer to the different proteins on the surface and the numbers refer to the subtype. These serotypes are also often given common names, usually from their region of origin, such as Influenza A "Hong Kong."

Influenza viruses are constantly changing or mutating slightly so that each flu season brings new serotypes that infect humans. Scientists make an educated guess each year on what serotypes of virus might occur the following year and formulate the following year's vaccine accordingly.

Influenza vaccine has been traditionally designed to protect against two subtypes of A and one subtype of B. There is also a "quadrivalent" vaccine available this year that protects against two subtypes of both A and B. Fortunately, the vaccines available for the 2013 - 2014 influenza season contain the influenza strains that are circulating in the wild.

It's important to note that receiving influenza vaccine each fall can't give you influenza. The vaccine is made up of killed virus particles that can't cause an infection. Rarely, some people do have a reaction to the vaccine with some fever and aches, but it is not an infection with the virus. The vaccine is commonly given in late fall which is when the common cold and other viral illnesses are being spread in the community. Often people catch a cold around the time of vaccine administration and blame the symptoms on the vaccine. People with egg allergy can also receive the vaccine if they don't have serious anaphylactic reactions to eggs. Those who do have serious egg allergy should receive the vaccine in a doctor's office or other facility equipped to treat severe reactions.

I also occasionally hear another concern from patients that receiving influenza vaccine will "weaken" their immune systems and they won't be able to fight the disease when they are exposed to it. Quite the opposite is true - by receiving the vaccine, you are exposing your immune system to the killed virus and your body will be prepared to react quickly to the virus if and when you are exposed to it. Without receiving the vaccine, it takes your body days to produce antibodies to fight off the infection which prolongs the illness and can also make you more susceptible to other infections like pneumonia.

Symptoms of influenza usually include rapid onset, fever (typically 100-103), dry cough, runny nose, chills, headache and body aches. Most people describe it as being hit by a truck. This is in contrast to the common cold that usually has a slow onset, low-grade fever (usually 100 or less), cough, runny nose and mild body aches. Both illnesses usually last around ten days.

Since influenza is caused by a virus, antibiotics are useless. There is anti-viral medication available to shorten the course of influenza, but it must be started in the first 48 hours of illness to be of any benefit. Many people choose not to take the medication since it is expensive and may only shorten the course by a day or two. The medication should be administered to high-risk individuals to help prevent infection if they have a high risk of exposure.

While it's best to contact your health care provider for advice on whether or not to be seen, most healthy people who develop symptoms of influenza usually do not need to see their doctor and should stay home to avoid spreading the infection to others. People who should see their physician include those with heart or lung conditions and children under age two. Those who have diabetes or weak immune systems should also be seen since they are more likely to develop secondary complications of influenza. Shortness of breath and dehydration with severe weakness are also indications to be seen.

If you get influenza, the best thing to do is stay home, drink plenty of fluids and get as much rest as possible. Over-the-counter cold medications offer some relief and pain relievers can also be helpful if you do not have an allergy or other medical problems that limit your ability to take them. Since dehydration is common with influenza, it's also very important to take only recommended doses of medications, particularly acetaminophen (Tylenol). As always, you should wash your hands frequently to avoid catching the flu from, or spreading it to others. Alcohol-based hand sanitizers are definitely not as effective as hand washing for flu prevention.