Why do we feel sick when we have the flu?

It all started with a slight irritation in the throat that turned into a persistent cough. The muscles in our body begin to ache, our tolerance wanes, and so does our appetite. The conclusion is undeniable: We are all victims of the flu. At first glance, it might seem logical to attribute this unpleasant mix of symptoms to the spread of infection in our bodies, but is that really the case? What, in fact, causes us to have such morbid feelings? Could it be that our own bodies are the instigators of this onslaught on our health?

The first time we get sick is when a pathogen, such as the flu virus, enters our system, infects and kills our cells. However, this unwanted invasion has an additional effect: it alerts our body’s immune system to our condition. Once an infection is detected, our body mounts its defenses. Cells called macrophages are on the front lines of the battle, tracking down and eliminating viruses and infected cells. The macrophages then release proteins called cytokines (some people call them cytosines, but that doesn’t sound right) whose mission is to recruit and organize more immune system cells to fight the virus. If this coordinated effort is strong enough, it will eliminate the infection before we know it. But this is just a prelude; in some cases, the virus spreads further, even reaching the bloodstream and vital organs. To avoid this sometimes dangerous fate, our immune system must mount a more vigorous attack, coordinating its activity with our brains.

This is where the unpleasant symptoms come into play, starting with sudden fever, pain, and drowsiness. So why are we going through all this? When the immune system is severely attacked, it secretes large amounts of cytokines, which trigger two responses. First, the vagus nerve extends throughout our body until it reaches the brain, where it quickly carries information to the brainstem, near key areas for pain processing. Second, the cytokines travel through our bodies until they reach the hypothalamus, the part of our brain that controls things like body temperature, thirst, appetite, and sleep. When the hypothalamus receives this message, it produces another molecule called prostaglandin E2 to prepare for battle. The hypothalamus sends signals instructing our muscles to contract and causes an increase in body temperature. Additionally, it induces sleep, causing us to lose appetite and thirst.

However, what is the purpose of all these uncomfortable symptoms? Although we still don’t know for sure, some theories suggest they contribute to our recovery. Warmer temperatures can slow the spread of germs, making it easier for our immune systems to destroy pathogens. Sleep allows our bodies to send more energy to fight infections. When we stop eating, our liver absorbs large amounts of iron from the blood, and since iron is essential for bacterial survival, this effectively weakens the bacteria. Reducing thirst can make us slightly dehydrated, reducing the spread of viruses through sneezing, coughing, vomiting, or diarrhea, but it’s important to note that lack of water can lead to dehydration, which can be dangerous.

Even physical pain can sensitize us, alerting us to wound infection, which can exacerbate or even cause complications in our condition. In addition to physical symptoms, the disease can cause irritability, sadness, and confusion. This is because cytokines and prostaglandins can reach higher structures in our brains and disrupt the activity of neurotransmitters such as glutamate, endorphins, serotonin and dopamine. This affects areas such as the limbic system, which monitors our emotions, and the cerebral cortex, which is involved in reasoning.

So, in reality, most of the discomfort we experience whenever we get sick is caused by our body’s own immune response. Unfortunately, this answer isn’t always perfect. Notably, millions of people around the world suffer from autoimmune diseases, in which the immune system interprets normal body signals as threats, so the body attacks itself. However, for most humans, millions of years of evolution have honed the immune system to work in our favor rather than against us. The symptoms of our diseases can be annoying, but taken together, they reflect an ancient process that will continue to protect our bodies from the outside world for centuries to come.

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