There are three types of influenza viruses: A, B, and C. Major epidemics are caused by A and B, and it is usually these two varieties that cause the flu. They have a diameter of about 100 nanometers, making them a medium sized virus. They have two kinds of spiky structures on their surface. One kind is a glycoprotein called hemagglutinin (HA), which is needed for the virus to bond to cells in the throat and bronchial tube and cause an infection, and the other is a glycoprotein called neuraminidase (NA), which has the function of breaking the linkages between HA and the cell.
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Symptoms usually develop after an incubation period of one to three days from the time of infection. The principal symptoms include a sudden fever, sore throat, headache, sore muscles, and a feeling of malaise. The fever lasts two to four days, and after that, respiratory symptoms such as a runny nose and cough will develop.
The flu in infants
Symptoms are varied, dominated by fever and respiratory symptoms such as a runny nose and cough.
The flu in children
In addition to fever and respiratory symptoms such as a runny nose and cough, cases involving abdominal symptoms such as nausea, stomach ache, and diarrhea, or neurological symptoms such as spasms and disorders of consciousness, as well as middle ear inflammation and muscle inflammation, are more common than in adults. The flu can also cause bronchitis in children. The fever may subside once during the course of the illness as if the flu had run its course, only to reappear half a day to a day later.
The flu in adults
The condition begins with a sudden high fever and then strong systemic symptoms such as sore throat, headache, joint pain, and malaise. The fever will subside two or three days after symptoms develop, but then respiratory symptoms such as a runny nose and cough will develop.
The flu in elderly people
Having weaker immune systems, elderly people who catch the flu and more likely to become seriously ill, and may develop secondary infections such as bacterial pneumonia. In particular, in patients with underlying disorders of the heart or lungs, the flu can lead to hospitalization or death.
If you have these symptoms, you should go to a hospital for an examination as soon as possible.