Causes and Risk Factors of H1N1 Swine Flu

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Influenza, or the flu, causes illness every year, usually between late fall and early spring in the Northern Hemisphere. There are many strains of the influenza virus, only some of which cause illness in humans. Other strains cause illness in other species such as pigs (swine), birds, dogs, and even bats.

H1N1 swine flu is a type of pandemic influenza that began circulating and caused a worldwide outbreak in 2009. It is caused by a specific strain of the influenza virus.

H1N1 swine flu causes and risk factors
 © Verywell, 2018


Occasionally, a strain of influenza that usually causes illness in one species of animal mutates and starts making humans sick. When this occurs, if transmission occurs easily between humans, it can cause a flu pandemic, even outside of the typical flu season. Since the early 20th century, flu pandemics have occurred approximately every 30 years.

H1N1 swine flu is a specific strain of the influenza virus that causes flu in pigs. In 2009, a new strain was found in humans that had not previously been seen. Officially, it is called influenza A (H1N1) pdm09 virus. This mutation and subsequent spread led to a pandemic that sickened millions of people worldwide and killed hundreds of thousands.

Risk Factors

Although anyone can get H1N1 swine flu, some people are more likely to be seriously affected by it. Seasonal influenza typically is most serious for older adults over the age of 65. However, the H1N1 swine flu pandemic disproportionately sickened and killed people under the age of 65.

According to the CDC

  • It's estimated that 80 percent of (H1N1)pdm09 virus-associated deaths were in people younger than 65 years of age.
  • During typical seasonal influenza epidemics, about 70 percent to 90 percent of deaths are estimated to occur in people 65 years of age and older.
  • H1N1 was also very serious for pregnant women.

It is believed that a majority of people over the age of 65 had some immunity to the H1N1 swine flu pandemic virus, which provided protection to those that would be at highest risk during a seasonal flu outbreak.

Current Risk

It's also important to note that the strain of influenza that caused the H1N1 swine flu pandemic is still circulating.

Although H1N1 does still cause seasonal influenza illness, the worldwide pandemic was declared over in August 2010 by the World Health Organization.

While H1N1 was new in humans at the time of the 2009 flu pandemic much of the world's population has now been exposed to it.

Frequently Asked Questions

  • Is the swine flu caused by bacteria?

    No, the swine flu is caused by (H1N1)pdm09—a strain of the influenza virus. It is not caused by bacteria. 

  • Who is at greater risk of complications from the swine flu?

    Adults under the age of 65, children, and pregnant women are at greater risk of complications from H1N1 swine flu. This is the opposite of traditional influenza strains where people ages 65 and older are at the greatest risk of complications and death.

  • What caused the swine flu pandemic?

    The swine flu originated in pigs and mutated to infect humans. It was first found in humans in 2009. This influenza strain spread around the world and caused a global pandemic. Millions of people became sick and hundreds of thousands of people died.

  • How can you prevent H1N1?

    The global H1N1 pandemic was declared over in August 2010. However, H1N1 is still circulating. You can prevent catching H1N1 by getting an annual flu shot. In addition, wash your hands frequently or use hand sanitizer and avoid indoor crowds during outbreaks of the flu.

1 Source
Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
  1. Centers for Disease Control and Prevention. H1N1 Pandemic (H1N1pdm09 virus).

Additional Reading

By Kristina Duda, RN
Kristina Duda, BSN, RN, CPN, has been working in healthcare since 2002. She specializes in pediatrics and disease and infection prevention.