This picture provides a 3D graphical representation of a generic influenza virion’s ultrastructure, and is not specific to a seasonal, avian or 2009 H1N1 virus. See PHIL 11824 for a view of this virus in which a portion of the virion’s protein coat, or “capsid”, has been cut away, revealing its inner nucleic acid core proteins, as well as a key identifying the organism’s protein constituents.

There are three types of influenza viruses: A, B and C. Human influenza A and B viruses cause seasonal epidemics of disease almost every winter in the United States. The emergence of a new and very different influenza virus to infect people can cause an influenza pandemic. Influenza type C infections cause a mild respiratory illness and are not thought to cause epidemics.

Influenza A viruses are divided into subtypes based on two proteins on the surface of the virus: the hemagglutinin (H), and the neuraminidase (N). There are 16 different hemagglutinin subtypes and 9 different neuraminidase subtypes. Influenza A viruses can be further broken down into different strains. Current subtypes of influenza A viruses found in people are influenza A (H1N1) and influenza A (H3N2) viruses. In the spring of 2009, a new influenza A (H1N1) virus emerged to cause illness in people. This virus was very different from regular human influenza A (H1N1) viruses and the new virus has caused an influenza pandemic.

Influenza B viruses are not divided into subtypes, however, influenza B viruses also can be further broken down into different strains.

CDC/ Douglas Jordan

This picture provides a 3D graphical representation of a generic influenza virion’s ultrastructure, and is not specific to a seasonal, avian or 2009 H1N1 virus. In this particular view, a portion of the virion’s protein coat, or “capsid”, has been cut away, revealing its inner nucleic acid core proteins. Note the key to the right identifying the virion’s protein constituents. For a fully intact, complete view of this virus, see PHIL 11825.

There are three types of influenza viruses: A, B and C. Human influenza A and B viruses cause seasonal epidemics of disease almost every winter in the United States. The emergence of a new and very different influenza virus to infect people can cause an influenza pandemic. Influenza type C infections cause a mild respiratory illness and are not thought to cause epidemics.

Influenza A viruses are divided into subtypes based on two proteins on the surface of the virus: the hemagglutinin (H), and the neuraminidase (N). There are 16 different hemagglutinin subtypes and 9 different neuraminidase subtypes. Influenza A viruses can be further broken down into different strains. Current subtypes of influenza A viruses found in people are influenza A (H1N1) and influenza A (H3N2) viruses. In the spring of 2009, a new influenza A (H1N1) virus emerged to cause illness in people. This virus was very different from regular human influenza A (H1N1) viruses and the new virus has caused an influenza pandemic.

Influenza B viruses are not divided into subtypes, however, influenza B viruses also can be further broken down into different strains.

CDC/ Douglas Jordan

This highly-magnified, digitally-colorized transmission electron micrograph (TEM) depicted numbers of virions from a Novel Flu H1N1 isolate.

What is 2009 H1N1 (swine flu)?  2009 H1N1 (sometimes called “swine flu”) is a new influenza virus causing illness in people. This new virus was first detected in people in the United States in April 2009. This virus is spreading from person-to-person worldwide, probably in much the same way that regular seasonal influenza viruses spread. On June 11, 2009, the World Health Organization (WHO) signaled that a pandemic of 2009 H1N1 flu was underway.

How does 2009 H1N1 virus spread?  Spread of 2009 H1N1 virus is thought to occur in the same way that seasonal flu spreads. Flu viruses are spread mainly from person to person through coughing or sneezing by people with influenza. Sometimes people may become infected by touching something – such as a surface or object – with flu viruses on it and then touching their mouth or nose.

What are the signs and symptoms of this virus in people?  The symptoms of 2009 H1N1 flu virus in people include fever, cough, sore throat, runny or stuffy nose, body aches, headache, chills and fatigue. Some people may have vomiting and diarrhea. People may be infected with the flu, including 2009 H1N1 and have respiratory symptoms without a fever. Severe illnesses and deaths have occurred as a result of illness associated with this virus.

CDC/ Cynthia Goldsmith

This digitally-colorized negative-stained transmission electron micograph (TEM) depicted a number of influenza A virions.

There are three types of influenza viruses: A, B and C. Human influenza A and B viruses cause seasonal epidemics of disease almost every winter in the United States. The emergence of a new and very different influenza virus to infect people can cause an influenza pandemic. Influenza type C infections cause a mild respiratory illness and are not thought to cause epidemics.

Influenza A viruses are divided into subtypes based on two proteins on the surface of the virus: the hemagglutinin (H) and the neuraminidase (N). There are 16 different hemagglutinin subtypes and 9 different neuraminidase subtypes. Influenza A viruses can be further broken down into different strains. Current subtypes of influenza A viruses found in people are influenza A (H1N1) and influenza A (H3N2) viruses. In the spring of 2009, a new influenza A (H1N1) virus emerged to cause illness in people. This virus was very different from regular human influenza A (H1N1) viruses and the new virus has caused an influenza pandemic.

CDC/ F. A. Murphy

This colorized transmission electron micrograph (TEM) revealed the presence of a number of Novel H1N1 virus virions in this tissue sample.

What is novel H1N1 (swine flu)?  Novel H1N1 (referred to as “swine flu” early on) is a new influenza virus causing illness in people. This new virus was first detected in people in the United States in April 2009. This virus is spreading from person-to-person worldwide, probably in much the same way that regular seasonal influenza viruses spread. On June 11, 2009, the World Health Organization (WHO) signaled that a pandemic of novel H1N1 flu was underway.

Why is novel H1N1 virus sometimes called “swine flu”?  This virus was originally referred to as “swine flu” because laboratory testing showed that many of the genes in this new virus were very similar to influenza viruses that normally occur in pigs (swine) in North America. But further study has shown that this new virus is very different from what normally circulates in North American pigs. It has two genes from flu viruses that normally circulate in pigs in Europe and Asia and bird (avian) genes and human genes. Scientists call this a "quadruple reassortant" virus.

CDC/ C. Goldsmith and D. Rollin

This transmission electron micrograph (TEM), taken at a magnification of 108,000x, revealed the ultrastructural details of two avian influenza A (H5N1) virions, a type of bird flu virus, which is a subtype of avian influenza A. At this magnification, one may note the stippled appearance of the roughened surface of the proteinaceous coat encasing each virion.

Although this virus does not typically infect humans, in 1997, the first instance of direct bird-to-human spread of influenza A (H5N1) virus was documented during an outbreak of avian influenza among poultry in Hong Kong. The virus caused severe respiratory illness in 18 people, of whom, 6 had died. Since that time, there have been other instances of H5N1 infection among humans.

During August to October, 2004, sporadic human cases of avian influenza A (H5N1) were reported in Vietnam and Thailand. Since December, 2004, a resurgence of poultry outbreaks and human cases has been reported in Vietnam. On February 2, 2005, the first of four human cases of H5N1 infection from Cambodia was reported. On July 21, 2005, the first human case of H5N1 in Indonesia was reported, and since then Indonesia has continued to report human cases in August, September, and October, 2005.

CDC/ Cynthia Goldsmith; Jackie Katz

Colorized transmission electron micrograph of Avian influenza A H5N1 viruses (seen in gold) grown in MDCK cells (seen in green).

Avian influenza A viruses do not usually infect humans; however, several instances of human infections and outbreaks have been reported since 1997. When such infections occur, public health authorities monitor these situations closely.

CDC/ Courtesy of Cynthia Goldsmith; Jacqueline Katz; Sherif R. Zaki

This negative stained transmission electron micrograph (TEM) shows recreated 1918 influenza virions that were collected from supernatants of 1918-infected Madin-Darby Canine Kidney (MDCK) cells cultures 18 hours after infection.

To separate these virions, the MDCK cells are spun down (centrifugation), and the 1918 virus in the fluid is immediately fixed for negative staining. The solid mass in lower center contains MDCK cell debris that did not spin down during the procedure. See PHIL 8160 for a black and white version of this micrograph.

Dr. Terrence Tumpey, one of the organization’s staff microbiologists and a member of the National Center for Infectious Diseases (NCID), recreated the 1918 influenza virus in order to identify the characteristics that made this organism such a deadly pathogen. Research efforts such as this, enables researchers to develop new vaccines and treatments for future pandemic influenza viruses.

The 1918 Spanish flu epidemic was caused by an influenza A (H1N1) virus, killing more than 500,000 people in the United States, and up to 50 million worldwide. The possible source was a newly emerged virus from a swine or an avian host of a mutated H1N1 virus. Many people died within the first few days after infection, and others died of complications later. Nearly half of those who died were young, healthy adults. Influenza A (H1N1) viruses still circulate today after being introduced again into the human population in the 1970s.

CDC/ Dr. Terrence Tumpey