Colds and the Flu - Introduction

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Flu Strains. A virus is a cluster of genes wrapped in a protein membrane, which is coated with a fatty substance that contains molecules called glycoproteins. Strains of the flu are identified according to the number of membranes and type of glycoproteins present.

Click the icon to see an image of a virus.

The two major flu strains are referred to as A and B:

• Influenza A is the most widespread and can infect animals and humans. Influenza A is the cause of the major pandemics of influenza that have occurred so far. It is usually further categorized by two subtypes based on two substances that occur on the surface of the viruses: hemagglutinin (H) and neuraminidase (N).
• Influenza B infects only humans. It is less common than type A, but is often associated with specific outbreaks, such as in nursing homes.

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The vast majority of flu cases are type A. Influenza A usually causes more severe disease than type B. There is some concern, however, that since influenza B has been less common in the past few years, some people, particularly small children, may have fewer antibodies to it and so may be at higher risk for severe infection.

H1N1 (Swine) Influenza

In April 2009, an outbreak of a new H1N1 virus, also known as swine influenza, began in Mexico and spread to the United States and other countries. Swine influenza is flu infection found in pigs. The virus that causes this infection in pigs can change (mutate) to infect humans. The disease is of concern to humans, who have no immunity against it. By June 2009, the World Health Organization had declared a worldwide swine flu pandemic, but by August 2010, the pandemic had ended.

The current strain of swine flu virus has been identified as H1N1. The virus is contagious and can spread from human to human. Symptoms in humans are similar to classic flu symptoms, which might include fever, cough, sore throat, headache, chills, fatigue, dizziness, lack of energy, diarrhea, and vomiting.

A safe and effective vaccine is now widely available, and was incorporated into the annual flu vaccine for the 2010 - 2011 season.