COLD AND FLU

Cold & Flu Center
Is it a cold or the flu?

Stuffy, runny nose, sore throat, hoarseness, and a cough. Do you have common cold symptoms? Read More About the Common Cold »

Fever, cough, stuffy, runny nose, headache, fatigue and muscle aches. Do you have flu symptoms? Read More About the Flu »

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What is the common cold, and what causes it?

The common cold, also known as a viral upper respiratory tract infection, is a contagious illness that can be caused by a number of different types of viruses. Because of the great number of viruses that can cause a cold and because new cold viruses develop, the body never builds up resistance against all of them. For this reason, colds are a frequent and recurring problem. In fact, on average, preschool children have nine colds a year, those in kindergarten, 12 colds a year; and adolescents and adults, seven colds per year.

What are the symptoms of the common cold?

Symptoms of a common cold include nasal stuffiness and drainage, sore throat, hoarseness, cough, and perhaps a fever and headache. Many people with a cold feel tired and achy. These symptoms typically last from three to 10 days.

How is the common cold spread?

The common cold is spread mostly by hand-to-hand contact. For example, a person with a cold blows or touches his or her nose and then touches someone else who then becomes infected with the virus. Additionally, the cold virus can live on objects such as pens, books, and coffee cups for several hours and can be acquired from such objects. While common sense would suggest that coughing and sneezing spread the common cold, these are actually very poor mechanisms for spreading a cold.

Does it have anything to do with exposure to cold weather?

Going out into the cold weather has no effect on the spread of a cold. The reason that there appears to be a relationship is that people spend more time indoors during the cold winter weather. In fact, however, it is the proximity to other people rather than the temperature outside that seems to be the culprit. For this same reason, children in daycare or kindergarten are particularly prone to having colds.

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.
Illustration of Influenza Virus

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.