Introduction:

Upper respiratory tract infections affect the airways in the nose, ears, and throat.

Structures of the throat include the esophagus, trachea, epiglottis, and tonsils.

The infections can be caused by viruses, bacteria, or other microscopic organisms. In most cases, these infections lead to colds or mild influenza (flu) and are temporary and harmless. In rare cases, flu can be severe, or the infections may turn into pneumonia.

Organisms that cause these upper respiratory tract infections are generally spread by:

• Direct contact (such as hand-to-mouth)
• Coughing or sneezing

The Common Cold

The common cold (medically known as infectious nasopharyngitis) is the most common upper respiratory tract infection. More than 200 viruses can cause colds. The most common cause is the rhinovirus, which is responsible for about half of all colds. Symptoms usually develop 1 - 3 days after being exposed to the virus.

A cold usually progresses in the following manner:

• It nearly always starts rapidly with throat irritation and stuffiness in the nose.
• Within hours, full-blown cold symptoms usually develop, which can include sneezing, mild sore throat, fever, minor headaches, muscle aches, and coughing.
• Fever is low-grade or absent. In small children, however, fever may be as high as 103°F for 1 or 2 days. The fever should go down after that time, and be back to normal by the 5th day.
• Nasal discharge is usually clear and runny the first 1 - 3 days. It then thickens and becomes yellow to greenish.
• The sore throat is usually mild and lasts only about a day. A runny nose usually lasts 2 - 7 days, although coughing and nasal discharge can persist for more than 2 weeks.

The adenovirus family causes upper respiratory infections (it is one of the many viruses that cause the common cold). It also causes pneumonia, conjunctivitis, and several other diseases. A newer strain of adenovirus has resulted in several deaths.

Influenza ("The Flu")

Every year, influenza strikes millions of people worldwide. Influenza epidemics are most serious when they involve a new strain, against which most people around the world are not immune. Such global epidemics (pandemics) can rapidly infect more than one fourth of the world's population. For example, the Spanish flu in 1918 and 1919 killed an estimated 20 million people in the U.S. and Europe and 17 million people in India. With modern society's dependence on air travel, an influenza pandemic could potentially inflict catastrophic damage on human lives, and disrupt the global economy.

The influenza virus mutates (changes) rapidly as it moves from species to species. Most Type A influenza strains (the most common strains) first develop in migratory waterfowl populations. While most avian influenza (bird flu) virus strains are relatively harmless, a few develop into "highly pathogenic avian influenza," which can be very deadly for domesticated poultry and livestock. As recent events have shown, these strains can also be deadly to humans. People can become infected by these bird flu strains through contact with contaminated chickens and pigs. The medical community is now greatly concerned about the H5N1 bird flu virus, which has infected and even killed people in several countries.

Symptoms of influenza. Patients usually feel sick 1 - 4 days after exposure to the influenza (flu) virus. The flu usually involves:

• Abrupt onset of severe symptoms, which include headache, muscle aches, fatigue, and high fever (up to 104°F).
• Cough (which is usually dry but can be severe) and sometimes a runny nose and sore throat.
• Children may experience vomiting, diarrhea, and ear infections, as well as other flu symptoms.
• The symptoms usually resolve in 4 - 5 days, although some people can experience coughing and feelings of illness for more than 2 weeks. In some cases, flu can become more severe or make other conditions worse.

Transmitting the Virus. The flu virus is spread primarily when a person with the flu coughs or sneezes near someone else. Adults with flu typically spread it to someone else from 1 day before symptoms start to about 5 days after symptoms develop. Children can spread the infection for more than 10 days after symptoms begin, and young children can transmit the virus 6 days or even earlier before the onset of symptoms. People with severely compromised immune systems can transmit the virus for weeks or months.

Flu Strains. A virus is a cluster of genes wrapped in a protein membrane, which is coated with a fatty substance that contains molecules called glycoproteins. Strains of the flu are identified according to the number of membranes and type of glycoproteins present.

The two major flu strains are referred to as A and B:

• Influenza A is the most widespread and can infect animals and humans. Influenza A is the cause of the major pandemics of influenza that have occurred so far. It is usually further categorized by two subtypes based on two substances that occur on the surface of the viruses: hemagglutinin (H) and neuraminidase (N).
• Influenza B infects only humans. It is less common than type A, but is often associated with specific outbreaks, such as in nursing homes.

The vast majority of flu cases are type A. Influenza A usually causes more severe disease than type B. There is some concern, however, that since influenza B has been less common in the past few years, some people, particularly small children, may have fewer antibodies to it and so may be at higher risk for severe infection.

H1N1 (Swine) Influenza

In April 2009, an outbreak of swine influenza began in Mexico and spread to the United States and other countries. Swine influenza is flu infection found in pigs. The virus that causes this infection in pigs can change (mutate) to infect humans. The disease is of concern to humans, who have no immunity against it. By June 2009, the World Health Organization had declared a worldwide swine flu pandemic.

The current strain of swine flu virus has been identified as H1N1. The virus is contagious and can spread from human to human. At this time, it is unknown how easily it can spread between people. Symptoms in humans are similar to classic flu-like symptoms, which might include fever, cough, sore throat, headache, chills, fatigue, dizziness, lack of energy, diarrhea, and vomiting.

The H1N1 flu outbreak in Mexico has resulted in several deaths in Mexico thus far. At least three death had been reported in the U.S. at the time of this writing. Officials were preparing for more.

Most people who get H1N1 flu will likely recover without needing medical care. Doctors, however, can prescribe antiviral drugs to treat people who become very sick with the flu or are at high risk for flu complications.

If you need treatment for H1N1 flu, the CDC recommends that your doctor give you zanamivir (Relenza) or osteltamivir (Tamiflu). These drugs work best if you receive them within 2 days of becoming ill. You may get them later if you are very sick or if you have a high risk for complications.

To prevent infection with H1N1 flu, people living in the same house as someone diagnosed with the virus should ask their doctor if they also need a prescription for these medicines. Careful respiratory hygiene and frequent hand-washing are also recommended steps for reducing the risk of getting H1N1 flu.

Avian Influenza (Bird Flu)

Although the risk of lethal viruses is generally low, scientists are greatly concerned about a particular virus called H5N1, which causes avian influenza. Since 1997, the H5N1 virus has triggered deadly outbreaks in poultry across Southeast Asia. As of December 16, 2008, 391 people had been infected with the bird flu in 15 countries. Of these people, 247 have died, according to the World Health Organization. No cases have been reported in the United States.

So far, the virus has spread from birds to humans. The virus does not seem to be easily spread from person to person. However, scientists and public health officials are monitoring the spread of H5N1 and working to contain it. Efforts include slaughtering infected birds, developing new vaccines, and stockpiling antiviral drugs such as oseltamivir (Tamiflu). Many poor nations have limited resources and already contend with other serious health problems, including HIV-AIDS. If H5N1 does mutate and spread, the consequences could be especially severe for these countries.

In April 2007, the FDA approved a vaccine to protect humans from avian influenza. Currently this vaccine is not being used for routine immunization. However, if the avian flu develops the ability to spread fairly easily from human to human, this vaccine may be made available.