The incubation period of this disease is generally 1-3 days (several hours to 4 days). In clinical practice, there may be sudden onset of high fever, with severe systemic symptoms but not severe respiratory symptoms, manifested as chills, fever, headache, fatigue, and overall soreness. The body temperature can reach 39-40oC and generally subsides gradually after 2-3 days. The overall symptoms gradually improve, but upper respiratory symptoms such as nasal congestion, runny nose, sore throat, and dry cough are more prominent. A few patients may have mild gastrointestinal symptoms such as nosebleeds, loss of appetite, nausea, constipation, or diarrhea. During the physical examination, the patient presented with an urgent appearance, flushed cheeks, mild conjunctival congestion and tenderness, pharyngeal congestion, and possible herpes on the oral mucosa. The lung was auscultated with only rough breathing, and occasional pleural friction sounds were heard. After the symptoms disappear, one still feels weak and weak, with poor mental state and slow physical recovery.
（1） There are three types of pulmonary complications that can occur
1. Primary viral pneumonia is relatively rare and was the main cause of the 1918-1919 pandemic. It is more common in patients with pre-existing heart and lung diseases (especially rheumatic heart disease and mitral stenosis patients) or pregnant women. Pulmonary lesions are mainly characterized by serous hemorrhagic bronchopneumonia, with extravasation of red blood cells, fibrous exudate, and formation of a transparent membrane. In clinical practice, there are symptoms such as persistent high fever, shortness of breath, cyanosis, coughing, and hemoptysis. Physical examination reveals low respiratory sounds in both lungs, full of wheezing sounds, but no signs of consolidation. The course of the disease can last for 3-4 weeks, with low white blood cell counts and reduced neutrophils. X-ray examination showed scattered flocculent shadows in both lungs. Patients may die due to heart failure or peripheral circulation failure. Both sputum and blood cultures are ineffective, with a high mortality rate.
2. Secondary bacterial pneumonia starts with simple influenza and worsens 2-4 days later, with increased severity and chills. Symptoms of systemic poisoning are obvious, with increased coughing, purulent sputum production, and chest pain. Physical examination shows that the patient has difficulty breathing, cyanosis, lungs full of rales, and signs of consolidation or focal pneumonia. White blood cells and neutrophils significantly increase, and influenza viruses are difficult to isolate. However, pathogenic bacteria can be found in sputum, with Staphylococcus aureus, pneumococcus, and Haemophilus being the most common.
3. Mixed pneumonia of virus and bacteria. Influenza virus and bacterial pneumonia coexist, with acute onset and persistent high fever. The condition is severe and can present as bronchopneumonia or lobar pneumonia. In addition to an increase in influenza antibodies, pathogenic bacteria can also be found.
（2） Extrapulmonary complications
Reye’s syndrome is a liver and nervous system complication of influenza A and B, and can also be seen in herpes zoster virus infection. This disease is limited to children aged 2 to 16 years old and can become an outbreak due to its association with influenza. In clinical practice, neurological symptoms such as nausea, vomiting, followed by drowsiness, coma, and convulsions may occur after a few days of acute respiratory infection. There may be liver enlargement but no jaundice, normal cerebrospinal fluid examination, no signs of encephalitis, elevated blood ammonia, mild liver function damage, and pathological changes. The brain only has cerebral edema and hypoxic neurodegeneration, with fat infiltration in liver cells. The cause is unknown, but in recent years it has been believed to be related to taking aspirin.
2. Toxic shock syndrome often occurs after influenza, accompanied by respiratory failure. Chest X-rays can show adult respiratory distress syndrome, but pneumonia lesions are not obvious. Influenza antibodies can rise in the blood, and pathogenic bacteria can be found in tracheal secretions, with Staphylococcus aureus being the most common.
3. Localized or systemic skeletal muscle necrosis in the rhabdomyolysis system, characterized by muscle pain and weakness, elevated serum creatine phosphatase and electrolyte disorder, may lead to acute renal failure.
