It starts with a tickle in the throat that turns into a cough. Your muscles start to ache, you become irritable, and you lose your appetite. It's official: you have the flu. It makes sense to assume that this miserable mix of symptoms is the result of the infection going through your body, but is it really? What makes you feel bad? What if your body itself leads this fierce attack?

You get sick for the first time when a pathogen like the flu virus enters your system, infecting and killing your cells. But this unwelcome intrusion has another effect: it alerts your body's immune system to your situation. As soon as it becomes aware of the infection, your body comes to your defense. Cells called macrophages take the lead on the attack, seeking out and destroying viruses and infected cells.Macrophages then release protein molecules called cytokines, whose job is to recruit and organize more cells that destroy viruses in your immune system. If this coordinated effort is powerful enough, it will clear the infection before you even know it.

But it's just your body that is setting the stage for real action. In some cases, viruses spread further, including in the blood and vital organs. To escape this dangerous fate, your immune system must launch a stronger attack, coordinating its activity with the brain. That is when unpleasant symptoms starts to appear, starting with a rise in temperature, body aches and drowsiness. So why are we experiencing  this effects?

MUST READ: Difference between normal anxiety — and what's an anxiety disorder?

When the immune system is seriously attacked, it secretes more cytokines, which triggers two responses. First, the vagus nerve, which runs through the body to the brain, quickly transmits information to the brainstem, passing near an important pain processing area. Secondly, cytokines travel through the body to the hypothalamus, the part of the brain responsible for controlling temperature, thirst, hunger, and sleep, among other things.When it receives this message, the hypothalamus produces another molecule called prostaglandin E2, which prepares it for war.

The hypothalamus sends signals that tell muscles to contract and cause body temperature to rise. appetite lost and thirst. But what is the point of all these unpleasant symptoms? Well, we're not sure yet, but some are theorizing that they help with recovery.

Raising temperatures can slow bacteria down and help the immune system destroy pathogens. When you sleep your body channels more energy to fight infections. When you stop eating, your liver can absorb much of the iron from the blood, and because iron is essential for bacteria to survive, it causes them to starve. Your reduced thirst makes you slightly dehydrated, reducing transmission through sneezing, coughing, vomiting, or diarrhea. While it's worth noting that if you don't drink enough water, dehydration can become dangerous. 


Body aches also make you more sensitive, calling attention to infected cuts that could make or even cause your condition. In addition to the physical symptoms, the disease or sickness can also make you irritable, sad, and confused. That is because cytokines and prostaglandins can reach even higher structures in the brain, disrupting the activity of neurotransmitters, such as glutamate, endorphins, serotonin, and dopamine.

It affects areas such as the limbic system, which oversees emotions, and the cerebral cortex, which is involved in reasoning. the body's immune response which causes much of the discomfort you feel whenever you get sick.

Unfortunately, it doesn't always work perfectly. In particular, millions of people around the world suffer from autoimmune diseases, in which the immune system processes it normal bodily signals as threats and then the body attacks itself.

 But for most of the human race, millions of years of evolution have refined the immune system to work for, rather than against us. 

The symptoms of our illnesses are bothersome, but collectively they point to an ancient process that will continue to barricade our bodies from the outside world for centuries to come.