Commonly known as “the flu” — influenza is an infectious disease of birds and mammals caused by RNA viruses of the family Orthomyxoviridae: the influenza viruses.
The most common symptoms:
- Runny nose
- Sore throat
- Muscle pains
- Severe headache
- Weakness and fatigue
- General discomfort
What is it?
Although it is often confused with other influenza–like illnesses (especially the common cold), influenza is a more severe disease caused by a different type of virus. Influenza may produce nausea and vomiting, particularly in children, but these symptoms are more common in the unrelated gastroenteritis, which is sometimes inaccurately referred to as “stomach flu” or “24-hour flu”.
Flu can occasionally lead to pneumonia, either direct viral pneumonia or secondary bacterial pneumonia, even for persons who are usually very healthy. In particular it is a warning sign if a child (or presumably an adult) seems to be getting better and then relapses with a high fever as this relapse may be bacterial pneumonia. Another warning sign is if the person starts to have trouble breathing.
How is it transmitted?
Typically, influenza is transmitted through the air by coughs or sneezes, creating aerosols containing the virus. Influenza can also be transmitted by direct contact with bird droppings, nasal secretions, or through contact with contaminated surfaces. Airborne aerosols have been thought to cause most infections, although which means of transmission is most important is not absolutely clear.
Influenza spreads around the world in seasonal epidemics, resulting in about three to five million yearly cases of severe illness and about 250,000 to 500,000 yearly deaths. These numbers rise to millions during some pandemic years. In the 20th century three influenza pandemics occurred, each caused by the appearance of a new strain of the virus, and killing tens of millions of people.
Often new influenza strains appear when an existing flu virus spreads to humans from another animal species, or when an existing human strain picks up new genes from a virus that usually infects birds or pigs. An avian strain named H5N1 raised the concern of a new influenza pandemic after it emerged in Asia in the 1990s, but it has not evolved to a form that spreads easily between people. In April 2009 a flu strain evolved that combined genes from human, pig, and bird flu. Initially dubbed “swine flu” and also known as influenza A/H1N1, it emerged in Mexico, the United States, and several other nations. The World Health Organization officially declared the outbreak to be a pandemic on 11 June 2009. The WHO’s declaration of a pandemic level 6 was an indication of spread, not severity, the strain actually having a lower mortality rate than common flu outbreaks.
Prevention and Treatment
Vaccinations against influenza are usually made available to people in developed countries. Farmed poultry is often vaccinated to avoid decimation of the flocks. The most common human vaccine is the trivalent influenza vaccine (TIV) that contains purified and inactivated antigens from three viral strains. Typically, this vaccine includes material from two influenza A virus subtypes and one influenza B virus strain. The TIV carries no risk of transmitting the disease. A vaccine formulated for one year may be ineffective in the following year, since the influenza virus evolves rapidly, and new strains quickly replace the older ones. Antiviral drugs such as the neuraminidase inhibitor oseltamivir (Tamiflu) have been used to treat influenza. However, their effectiveness is difficult to determine due to much of the data remaining unpublished.