The National Academies: What You Need To Know About Infectious Disease

The National Academies

What You Need To Know About Infectious Disease

A digitally colorized, negative-stained transmission electron micrograph depicting several influenza A virus particles.

Credit: F. A. Murphy/CDC


The “flu” has become a popular catch-all term to describe anything from a bad cold to stomach distress. But real flu, influenza, is the defined illness that many public health officials dread most. Each year in the United States about 36,000 people die from flu-related complications, and more than 200,000 are hospitalized.
The hardy influenza virus can survive on environmental surfaces, such as doorknobs and countertops, for 2 to 8 hours—one of the reasons that hand washing and surface hygiene is an important part of most flu control strategies.
Of even greater concern is a flu pandemic—a worldwide epidemic of a new strain of influenza virus from which the human population has no immunity. Depending on its severity, an influenza pandemic could result in 200,000 to 2 million deaths in the United States alone. In 2009 the WHO declared the current H1N1 “swine flu” a pandemic, although its human mortality rate to date has been relatively modest, with about 18,000 confirmed deaths worldwide.
How the Flu Spreads
Influenza viruses mainly spread when droplets from the cough or sneeze of an infected person are propelled through the air and land on the mouth or nose of someone nearby. Flu viruses may also spread when a person touches respiratory droplets on another person or on an object and then touches his or her own mouth or nose. The hardy influenza virus can survive on environmental surfaces, such as doorknobs and countertops, for 2 to 8 hours—one of the reasons that hand washing and surface hygiene is an important part of most flu control strategies.
Once the flu virus makes contact with mucous membranes in the eyes and nose, it heads to the cells along the upper respiratory tract, bronchial tubes, and trachea, where it swiftly multiplies. Scientists believe flu symptoms arise because growth of the virus damages the cells into which it has inserted itself and because the immune system, in trying to limit the damage, responds in ways that cause familiar discomfort: It sends out white blood cells that release chemicals called cytokines, causing muscle and joint pain, and it produces a fever, which is one of the body’s ways of mobilizing its defenses against invaders.
Seasonal Versus Pandemic Flu
The genius of the influenza virus lies in its ability to alter itself. The virus uses RNA rather than DNA as its genetic material. RNA viruses make frequent mistakes while copying themselves. Their high mutation rate means that RNA viruses evolve far more rapidly than DNA viruses, because every successive generation is a little different from the previous one. The flu virus’s surface proteins— hemagglutinin (H) and neuraminidase (N)—are also changeable. These proteins have a role in making it possible for a virus to invade and hijack cells. Hemagglutinin permits virus particles to gain access to the cell’s interior, and neuraminidase helps newly produced copies of the virus break free of the cell in quest of other cells to invade.
The extent and severity of a pandemic depend on the specific characteristics of the virus. While rare, pandemics sweep the world like wildfire.
There are three types of influenza viruses: A, B, and C. Only influenza A viruses are further classified by subtype on the basis of the H and N surface proteins. Influenza A subtypes and B viruses are further classified by strains. Among influenza A viruses, there are 16 known subtypes of hemagglutinin and 9 of neuraminidase. Many different combinations of these H and N proteins are possible, each representing a different subtype.
According to the CDC, the subtypes of influenza that are currently circulating among people worldwide include A H1N1, A H3N2, and B strains. Usually only one subtype predominates in a given flu season. Epidemics break out every year because of slight genetic mutations in a virus subtype’s surface proteins that result in a new strain of the virus—a process known as antigenic drift. New combination vaccines are formulated annually to protect against the three circulating strains of seasonal flu that experts predict will cause the most illness in the coming season.
Sometimes the virus’s surface proteins undergo a radical change—a process known as antigenic shift—resulting in an altogether new influenza subtype against which most humans have no immunity. The result can be a pandemic. The extent and severity of a pandemic depend on the specific characteristics of the virus. While rare, pandemics sweep the world like wildfire. In addition to the recent H1N1 pandemic, three major pandemics broke out in the 20th century: an H1N1 in 1918 (the misnamed “Spanish” flu), an H2N2 in 1957 (the “Asian” flu), and an H3N2 in 1968 (the “Hong Kong” flu). Of these pandemics, the 1918–1919 virus was the most fearsome, killing 50 million to 100 million people worldwide (or between 0.5 and 1 percent of the global population at that time). Many of those deaths were due to the effects of pneumococcal pneumonia, a secondary complication of flu for which no antibiotics existed in 1918. Diagnosis and treatment of this complication continue to be key to survival and recovery for flu patients.
Where Does Flu Virus Come From?
The source of all flu strains is migrating aquatic birds, such as wild ducks, geese, and terns. Domestic birds—such as chickens, geese, and ducks—also carry a large variety of flu strains. New flu strains enter human populations in several ways. Sometimes genetic material is exchanged between human and avian flu viruses when a human or other mammal is infected with both viruses. Often this mixing and matching of viral genes happens in pigs, which are uniquely susceptible to both human and avian flu viruses. The process of swapping genes is called “reassortment.” Today’s H1N1, for example, is a triple reassortment virus containing genetic materials from avian, swine, and human viruses.
The source of all flu strains is migrating aquatic birds, such as wild ducks, geese, and terns.
Another process, known as adaptive mutation, is more gradual. In this case, the longer an avian flu virus infects humans, the more it is able to bind to human cells as the virus adapts to its new host. Recent investigations suggest that the 1918 flu virus was a bird virus that became a human virus by slowly accumulating genetic mutations that helped it survive in a human host.
On rare occasions, flu viruses leap directly from birds to humans. In 1997 a highly fatal H5N1 bird flu broke out in Hong Kong, infecting 18 individuals and causing 6 deaths—a potential pandemic that was averted when authorities ordered the slaughter of more than 1.5 million domestic birds. If H5N1 were to acquire the ability to spread easily from person to person, a new influenza pandemic could be possible.
Vaccines provide an effective means to prevent infection by the influenza virus. But as the world learned with the 2009 H1N1 “swine flu” pandemic, making vaccine is a time-consuming process that normally takes 5 to 6 months. For more than 50 years, flu vaccine has been produced by injecting the whole virus into fertilized hens’ eggs. The virus is harvested, purified, chemically treated, and then weakened (attenuated) so that it cannot trigger infections. Despite appeals from scientists, governments have been slow to invest in newer and faster methods of producing flu vaccines, such as recombinant technologies or tissue-culture–based production methods that bypass the need to grow a whole virus in eggs or cells. However, recent moves by the U.S. government and vaccine manufacturers toward utilizing some of these new technologies provide hopeful signs. Scientists are also exploring ways to make a vaccine that is effective against all flu strains—a so-called universal vaccine. Such a development could dramatically improve the public’s protection against influenza infection.
How to Protect Yourself
  • Consider getting vaccinated against influenza. Vaccines are one of the best ways to reduce the morbidity and mortality associated with the disease. They do not themselves cause influenza in any form.

  • Cover your nose and mouth with a tissue when you cough or sneeze. Throw the tissue in the trash after you use it.

  • Wash your hands often with soap and water, especially after coughing or sneezing. Alcohol-based hand sanitizers are also effective.

  • Avoid touching your eyes, nose, or mouth, which can spread germs.

  • If you do get sick, stay home from work or school and limit your contact with others to keep from infecting them.

  • Ask your doctor whether you should take an anti-influenza drug, such as Tamiflu, which can be effective if taken within 48 hours of developing the symptoms of flu.

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What do you know about infectious disease?

Which are larger?

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    Bacteria are 10 to 100 times larger than viruses.

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    Bacteria are 10 to 100 times larger than viruses.

Infectious Disease Defined


The process by which regional economies, societies, and cultures are becoming integrated through a global network of trade, migration, communication, and the spread of new technology.

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