From time to time, a new disease emerges that could become a pandemic. This could be influenza or another virus.

A pandemic can happen when a new virus emerges that humans have not faced before. It is a disease of global proportions, covering a large geographical area. It is not seasonal, it may spread between continents, and it affects many people.

From 1918 to 1919, the so-called "Spanish flu" pandemic spread throughout the world, killing some 50 million people. Could it happen again?

Fast facts on flu pandemics:

Here are some key points about flu pandemics. More detail is in the main article.

  • A pandemic is a disease of global proportions that the human immune system is unable to deal with.
  • A flu pandemic could happen if a flu virus that has not affected humans before changes and starts infecting people.
  • When a pandemic happens, there is generally no cure, because the disease has changed quickly.
  • Following hygiene rules and keeping up with regular flu vaccinations reduce the risk of flu, and possibly of a pandemic.

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A flu pandemic could become a global threat if a virus mutates and spreads among people who have no resistance.

Influenza, or flu, is a contagious respiratory disease caused by a range of viruses. The effects vary from mild to serious, depending on the virus type and the health condition of the patient.

Symptoms include fever or chills, a cough, sore throat, runny or stuffy nose, body aches, headaches, and tiredness. It is a respiratory disease, and if it is severe, it can be fatal.

Influenza is classified as A, B or C.

  • Type A includes H1N1, commonly known as "swine flu," H5N1, also called "bird flu" or avian flu, and H3N2.
  • Type B does not have subtypes, but there are different strains.
  • Type C is considered mild, and unlikely to cause an epidemic.

In the United States (U.S.), annual flu vaccination is recommended, especially for young children, older people, and those with weakened immunity.

The more people are inoculated, the lower the risk of an epidemic, or widespread disease within a community.

What is the difference between an epidemic and a pandemic?

A flu epidemic happens when more cases of influenza than expected affect a particular population, at a specific time. Epidemics can happen when an illness is highly contagious, but the spread is controlled.

Factors that limit its spread are:

  • the population has an existing natural immunity
  • a certain proportion of the population is vaccinated
  • treatment is possible

A flue epidemic tends to be seasonal and limited to a specific geographical area. It can happen if an already existing subtype of the flu virus spreads. Seasonal flu epidemics are common in the U.S.

A flu pandemic is a disease of global proportions, covering a large geographical area. It may spread between continents, and it affects many people.

A pandemic is not seasonal. It occurs when a new subtype emerges among the human population that has not spread between people before.

It is likely to spread because:

  • the population does not have immunity
  • targeted medical therapy does not yet exist

A new virus can spread quickly if is highly contagious and can pass between people in a sustained manner. First, it will spread locally, then globally. A pandemic may result.

A flu virus that could lead to a pandemic is called an "influenza virus with pandemic potential."

Two "bird flu" viruses are H5N1 and H7N9. They are non-human, circulating in birds, but not generally among people.

Because of this, people have little or no immunity, but, very occasionally, one of these viruses affects a human.

However, if a virus changes, it could start to infect humans and spread easily among them. This could cause an influenza pandemic.

Previous flu pandemics have spread easily among people, causing serious and often fatal illness.

Past pandemics

The 20th century saw three influenza A virus pandemics. They all spread easily, became global within 12 months, and were often fatal:

  • Spanish flu, or A H1N1, killed over 500,000 people in the U.S. and perhaps 50 million globally between 1918 and 1919. Half of those who died were healthy, young people.
  • Asian flu, or A H2N2, killed about 70,000 Americans and millions of people worldwide from 1957 to 1958.
  • Hong Kong flu, or H3N1 resulted in 34,000 fatalities in the U.S. from 1968 to 1969.

The Asian and Hong Kong flus contained genes from a human influenza virus plus a bird flu virus.

More recently, strains of "bird flu" and "swine flu" have raised concerns about a possible pandemic.

Swine flu

Swine flu is a respiratory disease normally affecting pigs. Swine flu viruses do not normally infect humans, but sometimes they do.

A swine flu virus that infects a human is called a "variant virus." Variant swine flu viruses have been reported in the U.S.

Bird flu

Influenza A viruses can infect different animals, not only pigs and birds, but even whales and seals.

Certain subtypes of influenza A virus only occur in certain species, but birds are hosts to all known subtypes of influenza A viruses.

A virus that causes disease in one species can sometimes cross over to another one.

Until 1998, pigs in the U.S. were only affected by H1N1 viruses, but in 1998, a human H3N2 virus entered the pig population, causing widespread disease among swine. H3N8 viruses from horses have also passed into dogs.

A bird flu A virus could pass from animals to humans in one of two ways:

  • directly from birds or through contact with an environment that is contaminated with the virus, say, by working with birds
  • through an intermediate host, such as a pig

How does a virus change?

Influenza A viruses have eight separate gene segments. If two different types infect a person or animal at the same time, they can mix and create a new virus.

Imagine that a pig is infected with two influenza-A viruses at the same time, one human and one avian.

As the viruses replicate, the genetic information becomes mixed, and a new virus is created.

If this new virus then infects a human, the person will not have the immunity needed to resist it, because it is new. If it causes illness in people and is transmitted easily between them, and in a sustained manner, an influenza pandemic can occur.

Sometimes, a virus adapts gradually. If a continuous trickle of people become infected, the virus can mutate over time, increasing the chance of transmission between humans.

This gives scientists more time to find a solution to the problem.

Scientists see a pandemic of influenza or another virus as a possibility.

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People are advised to keep up their vaccinations against seasonal flu, although it is unlikely to protect against new strains.

They constantly monitor for these types of diseases, but if a virus changes, this makes it difficult to combat.

The task is made harder because:

  • it takes time to develop new therapies and ensure they are safe
  • scientists cannot predict what a new virus might do

In January 2016, scientists from the National Academy of Health issued a warning that a pandemic of some kind is possible.

They call on global authorities to prepare by:

  • strengthening public health as the first line of defense against a crisis
  • strengthening global and regional co-ordination
  • accelerating research and development into possible drugs, protective equipment and so on

For now, the best protection against a range of flu types is to avoid sources of exposure, to practice good hygiene, and to have the regular seasonal flu vaccine. However, existing flu vaccines are unlikely to protect against a pandemic flu, if it were to happen.