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Germs that infect humans

Influenza virus

Influenza virus

© Robert Alain, SME, INRS-Institut Armand-Frappier

Microorganism: the influenza virus belonging to the Orthomyxoviridae family

Disease: influenza or flu

Occurrence of the disease

History: numbers of pandemics (worldwide epidemics) of influenza have caused many deaths. Influenza epidemics are generally associated with mortality rates of ten to 20%; this can rise to more than 50% in “enclosed” populations, such as boarding schools. The pandemics of 1889, 1918, 1957, and 1968 were devastating. The pandemic of 1918-1919 caused 21 million deaths throughout the world.

Current situation: the World Health Organization (WHO) watches this virus very closely. WHO attempts to identify new strains of the virus in order to produce a vaccine each year that will reduce the likelihood of a new pandemic.

Mechanism of action of the microorganism: once it is in our lungs, the virus breaks down the lung’s protective coating, and attaches itself to the epithelial cells of the respiratory system. The symptoms of influenza are due to the death of epithelial cells.

Symptoms of the disease: flu symptoms are characterized by chills, headache, fever, and generalized muscle aches. The flu lasts between three and seven days, and symptoms similar to those of a cold develop. During this period, the fever drops. Normally, influenza is not fatal, but secondary bacterial infections (Staphylococcus aureus or Streptococcus pneumoniae) can cause death from pneumonia. Therefore, most deaths attributable to the influenza virus are in fact caused by secondary infections. These complications are more frequent in elderly people.

Incubation period: one to two days

Contagious period: the contagious period is from three to five days following the onset of symptoms in adults, and up to seven days after onset among children.

Hosts: humans, but especially birds and pigs; these may be the source of new infectious strains of influenza for humans.

Transmission: large quantities of virus are found in the nasal secretions of an infected person. The disease can therefore be spread by aerosols, that is, through the air.

Treatment: new antiviral medications are effective in reducing symptoms. Patients are advised to get plenty of sleep. One antiviral medication, amantadine, reduces the symptoms of type A influenza. However, this substance produces numerous side effects, such as insomnia and difficulty in concentrating.

Geographical distribution of the microorganism: worldwide

Prevention: since the 1940s, killed-virus vaccines (viruses incapable of infecting) have been used to control influenza. These vaccines are administered particularly to persons over the age of 65, as well as to those with chronic illnesses. The composition of the vaccine is reassessed each year.

Vaccine: each year, the inactivated vaccine is reformulated to ensure its effectiveness against the most dangerous strains of the virus identified by the WHO.

It is very difficult to produce vaccines each year that are effective against the influenza virus; this virus undergoes antigenic modification, that is, it changes its form and the immune system is no longer capable of recognizing it.