How severe is the flu on which day?

Influenza A, also known as Type A influenza, is a contagious viral infection. The critical period for disease progression in patients with Influenza A typically occurs on the 2nd to 5th day after infection. During this time, symptoms gradually worsen, particularly between the 3rd and 5th days, when the condition is usually more severe.

The progression of Influenza A typically includes four phases: the incubation period, the early stage of illness, the phase of symptom exacerbation, and the recovery phase. During the incubation period, patients may not exhibit obvious symptoms, but the virus is already replicating within the body. In the early stage of illness, patients may experience symptoms similar to those of a common cold, such as high fever, cough, sore throat, and runny nose. The body's immune system is actively fighting the virus, which can lead to an intensified inflammatory response and symptoms such as high fever, severe cough, and difficulty breathing.

The 2nd to 5th days after infection represent a critical period for disease progression, with the 3rd to 5th days typically being the most severe. The exact course of the illness may vary depending on individual factors. Patients with stronger immune systems or lower viral loads may experience a relatively shorter illness duration, while older adults, children, pregnant women, and individuals with chronic underlying medical conditions—who generally have weaker immune systems—may experience a prolonged course of illness. In some cases, serious complications such as pneumonia or respiratory failure may develop, requiring a longer recovery period.

At home, maintaining good air circulation, using masks, and regularly cleaning and disinfecting frequently touched surfaces can help reduce the risk of virus transmission.