Why is viral infection often accompanied by fever symptoms?

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Since the outbreak of COVID-19, daily monitoring of body temperature has become a daily practice. In fact, not only the COVID-19 epidemic, but also when influenza, SARS and other diseases spread in the past, taking body temperature is a simple way for many people to judge whether they are infected with the virus. This approach is not entirely unreasonable.

Among human organs, skeletal muscle and liver are the main heat-generating organs, and the skin is the radiating organ. These two organs coordinate the generation and dissipation of heat, and then control the body temperature at around 37°C. Viruses and bacteria will generate exogenous heat after entering the human body. At this time, the white blood cells in the human body will also produce cytokines to attack bacteria and viruses. The endogenous heat they generate during the attack will also cause the body temperature to rise.

That is to say, when viruses and bacteria invade the human body, the human body will initiate a defense mechanism, and fever is a signal released by the body. In the past, many people attributed this signal from the body to the immune system, believing that the immune system automatically turned on its defenses. However, recently, researchers at Harvard University found that this phenomenon occurs mainly through the regulation of the brain.

Through experiments, the researchers found a group of special brain cells in mice that coordinate a series of behavioral changes, such as fever, chills, and loss of appetite, after viruses or bacteria invade the human body. The researchers then managed to give mice a fever due to the bacterial infection and searched their brains for neurons that were activated at this time, eventually finding neurons in the ventromedial preoptic area (VMPO) of the hypothalamus.

Using methods such as chemical genetics and optogenetics, the researchers controlled an additional 1,000 VMPO neurons. It was found that after activating these cells, the mice developed a series of symptoms very similar to those of humans after infection, including increased body temperature, fatigue, chills, and decreased appetite. This presumably established that VMPO neurons play a key role in signaling bacterial and viral invasion of the body.

Remarkably, these neurons are right next to the blood-brain barrier. In further research, the scientists found that this position is important. It was finally determined that the cells that make up the blood-brain barrier are in contact with the blood and the peripheral immune system. Once activated, they secrete cytokines and chemokines, which effectively activate the neurons found in the hypothalamus, eventually causing fatigue, increased body temperature, Symptoms such as loss of appetite.

Through this research, people have a more comprehensive understanding of virus invasion, fever, etc.