Why Flu Can Be Deadly

Author: Chen Dianjie, Fifth Medical Center, PLA General Hospital

Reviewer: Zhang Xin, Deputy Chief Physician, Fifth Medical Center, PLA General Hospital

In our country, almost no one talks about "deaths from influenza."

But in fact, many people who died from myocardial infarction, pneumonia, encephalitis, myocarditis or septic shock had their illness and sudden death triggered by influenza.

Influenza (flu for short) is an acute respiratory infection caused by influenza virus. It is also a highly contagious and fast-spreading disease. Typical clinical symptoms are: sudden onset of high fever, body aches, significant fatigue and mild respiratory symptoms.

Figure 1 The difference between influenza and the common cold

Influenza can be divided into three types: A, B, and C. Influenza A viruses often undergo antigenic mutations, are highly contagious, spread rapidly, and are very likely to become a large-scale epidemic. Influenza A (H1N1) is a type of influenza A.

Although influenza is a self-limiting disease and sometimes heals on its own without treatment, its harm far exceeds that of other respiratory infectious diseases. Infants, the elderly, and patients with underlying heart and lung diseases are especially prone to serious complications such as pneumonia, which can lead to death.

In 1918, the "Spanish flu" pandemic caused by the influenza A (H1N1) virus swept across the Americas, Europe, Asia, and even Inuit communities, infecting more than 500 million people worldwide and killing 50 to 100 million people. It was the most serious epidemic in human history.

Even today, more than 100 years later, humans have not completely defeated influenza. In the United States, during the last severe flu season (2014/2015), more than 35 million people fell ill from influenza and more than 50,000 died.

During the peak of influenza in late 2017 and early 2018, several cities in my country (Beijing, Shanghai, Nanjing, Shijiazhuang, etc.) saw a single children's hospital see more than 10,000 outpatient visits per day. There was also a phenomenon of people "frantically rushing for Tamiflu" and "Tamiflu sold out". (Tamiflu: The trade name of oseltamivir phosphate in China, a specific inhibitor of neuraminidase that can inhibit mature influenza viruses from leaving host cells, thereby inhibiting the spread of influenza viruses in the human body to treat influenza.)

Although the mortality rate of seasonal influenza today is less than 0.1% compared to the 1918 influenza pandemic (the mortality rate of infected people was over 2.5%), influenza is still one of the infectious diseases that cannot be ignored.

The reason why influenza can be fatal is related to the body's immune response after the influenza virus enters the human body.

Step 1 : After the influenza virus enters the human body, it begins to replicate rapidly and damage the body's epithelial cells. Faced with foreign "invaders", white blood cells, as the body's "guards" against pathogens, immediately go into battle - they attack the influenza virus as soon as they see it, and in many cases they even accidentally injure "their own people".

After the smoke of gunpowder cleared, influenza viruses were everywhere, and epithelial cells also suffered heavy losses. When epithelial cells are damaged, the body will show symptoms such as respiratory congestion and edema, increased secretions, sore throat, nasal congestion, runny nose, and cough.

Figure 2 (copyright image from the gallery, unauthorized reproduction)

Step 2 : While the white blood cells are "firing continuously", the influenza virus is also "reproducing endlessly" and continuing to replicate. Seeing that the war has entered a stalemate, the white blood cells decide to escalate the war and completely eliminate the influenza virus, so they come up with a plan to raise the body temperature. At this time, the human body shows symptoms of fever. The influenza virus is not resistant to high temperatures and cannot replicate in a high temperature environment. When the war reaches this stage, the white blood cells will generally be the winner, which means that the influenza will soon be cured.

Therefore, when influenza patients have fever symptoms, do not rush to cool them down. Clinically, it is recommended that if the body temperature of influenza patients does not exceed 38.5℃, physical cooling is generally used to relieve the patient's fever and discomfort.

Figure 3 (copyright image from the gallery, no permission to reprint)

Step 3 : If the fever persists and the white blood cells are unable to defeat the virus, or even to suppress the crazy replication of the flu virus, then the next step will be entered. The white blood cells are determined to perish together with the flu virus. So the white blood cells activate the self-destruct device - the cytokine storm, and the maximum amount of cytokines is released, which can even destroy the entire body.

In this case, whether the virus can be eliminated depends on the individual. But what is certain is that many organs will suffer fatal attacks. Patients with mild symptoms will develop pneumonia, while those with severe symptoms will develop critical conditions such as acute respiratory distress syndrome, shock, and multiple organ failure. This is the real reason why the influenza virus can be fatal in a short period of time.