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Swine Flu

Last Updated on Jul 28, 2021
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Swine influenza or 'Swine flu' is a type of influenza caused by a new strain of the H1N1 Type A influenza virus that has originated from the pigs but transmitted mostly from human to human.

Swine influenza or 'Swine flu' is a type of influenza caused by the Type A swine influenza virus (SIV), that commonly infects pigs.

During the 1918 flu pandemic, pigs and humans became sick and it was then that the connection came to be clearly noticed. For the next 60 years, H1N1 had a unique dominance as the cause of swine flu. Between 1997 and 2002, influenza-causing new strains emerged in North America, some of which comprised of reassorted genes derived from human, swine and avian viruses. These strains of viruses have grown to be the major cause of swine flu in Northern America.

Swine Flu

There was a massive effort at vaccination after the 1918 pandemic but it was plagued by reports of developing a neurological condition that leads to weakness of muscles, called Guillain-Barre syndrome, that sometimes had fatal outcomes.

The name influenza is derived from Italian word 'influenza', that means "influence" (Latin: influentia). The word 'flu' is derived from the word 'fluc' that means 'airway passage' of lungs.


The major flu outbreaks in the world have included the Spanish Flu, Asian flu and the Hong Kong Flu.

Flu pandemics over last 100 years

YearInfluenza A virus SubtypePeople Infected (approx)Deaths
1918-19Spanish FluH1N1500 Million50 Million
1957 Asian FluH2N2 2 Million
1968-69Hongkong FluH3N21 Million
2003-2009Bird Flu H5N1 424261

Pigs are unique in that they can act as hosts for strains of influenza virus that infect three different species- humans, birds and needless to say, pigs! These strains of viruses exchange their genes, while in the pig, to give rise to new and dangerous combination of viral strains. In China and in Vietnam the avian influenza virus H3N2 is also found in pigs. The avian strain H5N1 has also been detected in pigs in China. This has resulted in fears over a new strain of virus against which the human race has no immunity and can, therefore, cause innumerable deaths. The swine population becomes a potent reservoir of influenza viruses which can bid their time and attack humans when the latter’s immunity is low.

It must be noted that direct transmission from pigs to humans is extremely rare and that only 12 cases have been reported in the U.S.A since 2005. Transmission of SIV from the pigs to humans can however result in the formation of antibodies against the virus resulting in a condition called zoonosis.

In humans, ‘swine flu’ is known to be caused by H1N1, H1N2, H3N1, H3N2 and H2N3 subtypes of the type A influenza virus, while in pigs the disease is caused by the widely prevalent strains H1N1, H3N2, and H1N2.


The recent 2009 flu outbreak is due to a new strain of subtype H1N1 not previously reported in pigs. Analysis has revealed that two strains of the subtype H1N1 are involved in the formation of this new strain . The origin of this new strain is still being investigated. It is alleged to have evolved in the eggs that scientists use to develop vaccines. But what makes it potent is that it is not a zoonotic infection but a human to human transmittable disease.

Swine influenza is common in the midwestern United States, Mexico, Canada, South America, Europe (UK, Sweden, Italy), Kenya, Mainland China, Taiwan, Japan and other regions of eastern Asia.

There has been a global preparedness to tackle the swine flu thanks to the measures taken against avian flu (or bird flu) that affected several parts of the world. US President Barack Obama has said “We are closely monitoring the emerging cases of swine flu in the United States, and this is, obviously, a cause for concern and requires a heightened state of alert. But it’s not a cause for alarm.”


The World Health Organisation issued a statement that there is currently no vaccine for checking swine flu and it would take not less than six months to develop such a vaccine. The vaccine is reported to be ready by June 2009.

Influenza virus A- flu in humans and pigs, common in pigs
Influenza virus B- flu in humans, not reported in pigs
Influenza virus C –flu in humans, pigs, rare in pigs, not reported in birds


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Latest Publications and Research on Swine Flu

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This is an informative article on this subject and I really enjoyed the read. My regards and appreciation to the writer of this amazing material... To know more about Flu Symptoms


I am so worried about catching the swine flu. I have done everything in my power to protect myself and my family. Even to the point that if it were to become so out of control that we needed to be locked in our house for some time due to the pandemic.


Very good illustrations by Medindia.net. Dr.GANGADHAR RAO HYDERABAD


An interesting Abstract from a recent paper on Swine Flu

Novel Swine-Origin Influenza A (H1N1) Virus Investigation Team

Background - On April 15 and April 17, 2009, novel swine-origin influenza A (H1N1) virus (S-OIV) was identified in specimens obtained from two epidemiologically unlinked patients in the United States. The same strain of the virus was identified in Mexico, Canada, and elsewhere. We describe 642 confirmed cases of human S-OIV infection identified from the rapidly evolving U.S. outbreak.

Methods - Enhanced surveillance was implemented in the United States for human infection with influenza A viruses that could not be subtyped. Specimens were sent to the Centers for Disease Control and Prevention for real-time reverse-transcriptase–polymerase-chain-reaction confirmatory testing for S-OIV.

Results- From April 15 through May 5, a total of 642 confirmed cases of S-OIV infection were identified in 41 states. The ages of patients ranged from 3 months to 81 years; 60% of patients were 18 years of age or younger. Of patients with available data, 18% had recently traveled to Mexico, and 16% were identified from school outbreaks of S-OIV infection.

The most common presenting symptoms were fever (94% of patients), cough (92%), and sore throat (66%); 25% of patients had diarrhea, and 25% had vomiting. Of the 399 patients for whom hospitalization status was known, 36 (9%) required hospitalization. Of 22 hospitalized patients with available data, 12 had characteristics that conferred an increased risk of severe seasonal influenza, 11 had pneumonia, 8 required admission to an intensive care unit, 4 had respiratory failure, and 2 died. The S-OIV was determined to have a unique genome composition that had not been identified previously.

Conclusions A novel swine-origin influenza A virus was identified as the cause of outbreaks of febrile respiratory infection ranging from self-limited to severe illness. It is likely that the number of confirmed cases underestimates the number of cases that have occurred.

Raja Ramachandran, Senior Resident Nephrology, PGIMER, Chandigarh,INDIA

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