Infectious Diseases - Influenza

Influenza (the flu) is a contagious respiratory disease caused by a virus. When a person infected with the flu sneezes, coughs, or even talks, the virus is expelled in droplets into the air and may be inhaled by anyone nearby. It also can be transmitted by direct hand contact. The flu primarily affects the lungs, but the whole body experiences symptoms. The infected person usually becomes acutely ill, with fever; chills; weakness; loss of appetite; and aching muscles in the head, back, arms, and legs. The person with influenza infection also may have a sore throat, a dry cough, nausea, and burning eyes. The accompanying fever increases quickly—sometimes reaching 104 degrees—but usually subsides after two or three days. Influenza leaves the patient exhausted.

For healthy individuals, the flu is typically a moderately severe illness, with most adults and children back to work or school within a week. For the very young, the very old, and people who are not in good general health, however, the flu can be very severe and even fatal. Complications such as secondary bacterial infections may develop, taking advantage of the body's weakened condition and lowered resistance. The most common bacterial complication is pneumonia, but sinuses, bronchi (lung tubes), or inner ears also can become secondarily infected with bacteria. Less common but very serious complications include viral pneumonia, encephalitis (inflammation of the brain), acute renal (kidney) failure, and nervous system disorders. These complications can be fatal.

Who Gets the Flu?

Anyone can get the flu, especially if there is an epidemic in the community. (An epidemic is a period when the number of cases of a disease exceeds the number expected based on past experience.) During an epidemic year, 20–30 percent of the population may contract influenza. Not surprisingly, people who are not healthy are considered at high risk for flu and its complications. The high-risk population includes those who have chronic lung conditions, such as asthma, emphysema, chronic bronchitis, tuberculosis, or cystic fibrosis; those with heart disease, chronic kidney disease, diabetes, or severe anemia; people residing in nursing homes; those older than age sixty-five years; and some health care workers.

Vaccines

Influenza can be prevented by inoculation with a current influenza vaccine, which is formulated annually so that it contains the influenza viruses expected to cause the flu the next year. The viruses are killed or inactivated to prevent those who are vaccinated from getting influenza from the vaccine. After being immunized, the person develops antibodies to the influenza viruses. The antibodies are most effective after one or two months. High-risk people should be vaccinated early in the fall because peak flu activity usually occurs around the beginning of the new calendar year. The flu season usually runs from October to May and peaks in December and January.

Each year's flu vaccine protects against only the viruses that were included in its formulation. If another strain of flu appears, people still can catch the new strain although they were vaccinated for the primary expected strains. The 2003–2004 flu season was one of the worst in recent memories, with a nationwide shortage of vaccine early in the season, a time when the virus was peaking, and children were dying from the illness (at least 142 individuals under eighteen years old).

Most people have little or no reaction to the vaccine; 25 percent may have a swollen, red, tender area where the vaccination was injected. Children may suffer a slight fever for twenty-four hours or have chills or a headache. Those who already suffer from a respiratory disease may experience worsened symptoms. Usually, these reactions are temporary. Because the egg in which the virus is grown cannot be completely extracted, people with egg protein allergies should consult their physicians before receiving the vaccine and, if vaccinated, should be closely observed for any indications of an allergic reaction.

Avian Influenza

Avian influenza, or bird flu, is an infectious disease of birds caused by type A strains of the influenza virus. The disease, which was first identified in Italy more than one hundred years ago, occurs worldwide. An outbreak of bird flu has affected bird populations in countries throughout Asia. It has affected humans as well—as of March 2004, the death toll from avian influenza was twenty-four people. According to the CDC, it is believed that the cases in humans resulted from contact with infected birds or surfaces contaminated with excretions from infected birds. The disease usually only affects birds and pigs; the first documented infection of humans occurred in Hong Kong in 1997.

Because these viruses usually do not infect humans, there is little or no immune protection against them. If an avian influenza virus infected people and gained the ability to spread easily from person to person an "influenza pandemic" could begin. To help prevent this from occurring, sick and exposed birds are killed and infected people are treated and isolated.