Flu
(Influenza)
Medical Author: Charles Davis, MD, PhD
Medical Editor: Melissa Conrad Stöppler, MD
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What to Do if You Get the Flu
Medical Author: Melissa Conrad Stöppler, MD
Medical Editor: William C. Shiel Jr., MD, FACP, FACR
 Influenza, or the "flu," is a illness of the breathing
system (respiratory system) and muscles caused by a virus. While a vaccine is
available to prevent the flu, its effectiveness varies according to the degree
of match between the viral strains used to prepare the vaccine and those strains
actually in circulation in a given year. Not everyone receives the flu vaccine, and even
some of those who do can develop symptoms of the flu.
Mild cases of the flu may seem like common colds. But most cases of the flu
can be distinguished from colds because the symptoms (cough, muscle aches and
pains, sore throat, fatigue, and headache)
are more severe than those of the common cold. Flu symptoms also tend to occur suddenly and include high fevers
(temperatures of 101 degrees F or more). In children, fevers are typically even
higher than those in adults.
The flu is a serious illness that can be fatal in people
whose immune systems are weakened, the elderly, and those with chronic medical
conditions. Even healthy people who develop the flu cannot work, attend school,
or participate in normal activities for several days. Complications of the flu
can develop in anyone and include pneumonia, ear infections, sinus infections,
or bronchitis.
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What is influenza?
Influenza, commonly called "the flu," is an illness caused by viruses
that infect the respiratory tract. Compared with most other viral
respiratory infections, such as the common cold, influenza (flu) infection often causes a more severe illness with a mortality rate (death rate) of about 0.1% of people
who are infected with the virus. Unusually severe worldwide outbreaks (pandemics) have occurred several times in the last 100 years since influenza virus was identified in 1933. By an examination of preserved tissue, the worst influenza pandemic occurred in 1918 when the virus caused between 40 to 100 million deaths with a mortality rate estimated to range from 2% to 20%.
Haemophilus influenzae is a bacterium that was incorrectly considered to cause the flu until the virus was demonstrated to be the correct cause in 1933. This bacterium can cause lung infections in infants and children, and
it occasionally causes ear, eye, sinus, joint, and a few other infections, but not the flu.
What are the causes of the flu?
The flu (influenza) viruses
Influenza viruses are divided into three types, designated A, B, and C.
Influenza types A and B are responsible for epidemics of respiratory
illness that occur almost every winter and are often associated with
increased rates of hospitalization and death. Influenza type C differs
from types A and B in some important ways. Type C infection usually causes
either a very mild respiratory illness or no symptoms at all; it does not
cause epidemics and does not have the severe public health impact of
influenza types A and B. Efforts to control the impact of influenza are
aimed at types A and B, and the remainder of this discussion will be
devoted only to these two types.
Influenza viruses continually change over time, usually by mutation (change in the viral RNA). This constant changing often enables the virus to evade the immune system of the host (humans, birds, and other animals) so that the host is susceptible to changing influenza virus infections throughout life. This process works as follows: a host infected with influenza virus develops antibody against that virus; as the virus changes, the "first" antibody no longer recognizes the "newer" virus and reinfection can occur. The first antibody may in some instances provide partial protection against reinfection with an influenza virus.
Type A viruses are divided into types based on differences in two viral surface proteins called the hemagglutinin (H) and the neuraminidase (N). There are 16 known H subtypes and
nine known N subtypes. These surface proteins can occur in many combinations. When spread by droplets or direct contact, the virus, if not killed by the host's immune system, replicates in the respiratory tract and damages host cells.
Antigenic shift and drift
Influenza type A viruses undergo two kinds of changes. One is a series
of mutations that occurs over time and causes a gradual evolution of the
virus. This is called antigenic "drift." The other kind of
change is an abrupt change in the hemagglutinin and/or the neuraminidase
proteins. This is called antigenic "shift." In this case, a new
subtype of the virus suddenly emerges. Type A viruses undergo both kinds
of changes; influenza type B viruses change only by the more gradual
process of antigenic drift and therefore do not cause pandemics.
Next: What are flu symptoms? »
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