All persons infected with influenza virus may not necessarily have symptoms and those with the symptoms listed above will not all have the flu. Many viruses and bacteria can cause diseases that are indistinguishable from the flu.
Between 40% and 85% of individuals infected with influenza virus develop the disease. Among others the disease will go unnoticed. This is linked in part to the person’s age and background immunity. If the person has been infected by a flu virus with a similar strain, it is likely that his symptoms are reduced or even pass unnoticed.
According to a 2004 study with 7164 people in Europe, North America and in the southern hemisphere, for all age groups, the probability of being infected with influenza reduced if certain symptoms are absent:
- Without fever: 40% less chance that the symptoms are caused by the influenza virus
- In the absence of cough: 42% less chance that the symptoms are caused by the influenza virus
- In the absence of nasal congestion: 49% less chance that the symptoms are caused by the influenza virus.
We also observed that for people aged over 60 years, the probability of being infected with influenza is increased 5.4 times in the presence of:
- Rapid onset of symptoms
- Cough
- Fever.
At the same persons over 60 years the risk that the symptoms are caused by influenza is 5 times higher in the presence of:
- Fever
- Cough.
Also among those over 60 years:
- Fever isolation increases the risk of suffering from the flu by 3.8 times,
- Chills increase the risk of being infected with the flu by 2.6 times.
- Discomfort only: to increase the risk of being infected with the flu by 2.6 times.
It was also noted that if the sneezes are present the risk that the symptoms are caused by influenza is decreased by 47%.