There are three types of influenza – A, B and C – but types A and B occur much frequently in human beings than type C.

Type A

The influenza A virus is found in waterfowls and waders and can be transmitted to people. There are three types, H1N1, H2N2 and H3N2, which are also transmitted from person to person. Type A is the most common form of influenza in Denmark.

The virus can mutate from one year to the next and make people seriously ill, as we have seen it with type A (H1N1), or type A (H5N1), known as the bird flu.

In the Danish Medicines Agency, we have compiled answers to a series of questions and answers about the A virus (H1N1).

Type B

The type B virus only circulates among human beings, and it can make people ill in a geographically limited area, often when the type A influenza is subsiding. Type B influenza breaks out less frequently than influenza A, so when people fall ill with influenza B, it is often more serious because only a few will have developed antibodies.

Influenza has many faces

Seasonal influenza

Seasonal influenza spreads easily and breaks out every winter. Every second or third year, seasonal influenza can cause an epidemic, infecting up to one million people in Denmark in only a few weeks' time (Denmark's population = 5.5 million).

Bird flu

Bird flu is a disease which rarely infects people, but often breaks out in birds. For more than a decade, a particularly virulent form (H5N1) has made millions of birds ill across the globe, and in specific cases, it has infected people, causing a lot of anxiety, severe disease and fatal outcomes.

Influenza pandemic

When an epidemic is spreading worldwide, it is called a pandemic. It spreads when a new type of influenza virus emerges, different from the viruses the world has seen before. It spreads because people have no immunity to it.

It is impossible for doctors to predict when a pandemic will break out and how dangerous it will be. During the outbreak of a pandemic, children and adolescents are more susceptible to the virus, and most influenza cases are reported among the 11 to 14-year-olds. People's generally good health and vaccinations have reduced the number of deaths significantly since the first known pandemic.

Pandemics

In the previous century, we know of three pandemics, and 2009 marked this century's first pandemic.

• 1918: The Spanish flu, causing the death of 20 to 50 million people
• 1957: The Asian flu, causing the death of 1.7 million people
• 1968: The Hong Kong flu, causing the death of 1.3 million people
• 2009: Influenza A (H1N1): The World Health Organization WHO has received reports of 16,000 deaths: WHO on influenza 2009.