Humans evolved a robust inflammatory response to fight constant threats like infections. In today's relatively sterile world, this powerful system lacks its historical targets and can overreact to modern triggers, leading to the chronic low-level inflammation that is at the heart of many modern diseases.

The hygiene hypothesis suggests that modern sterile environments leave our evolved immune systems 'bored.' Lacking traditional targets like germs and worms, these systems can overreact to harmless substances like gluten, leading to a rise in food intolerances and autoimmune issues.

The rise in consumer cleaning products and spick-and-span households reduces our exposure to diverse microbes. According to the hygiene hypothesis, this lack of immune system training can make our bodies less robust and more prone to overreacting to benign substances like food proteins, thus fostering allergies.

Our immune systems evolved to mount robust inflammatory responses against acute threats like infections and traumas. In the modern world, which lacks these constant threats, this same sensitivity causes our bodies to overreact to environmental triggers. This evolutionary mismatch creates the chronic, low-level inflammation that drives modern diseases.

DNA mutation is not always harmful; it is essential for a healthy immune system. A process called somatic hypermutation allows immune cells to reshuffle their DNA to create effective antibodies. Vaccines work by stimulating this mutation process, training the body to fight specific pathogens.

The modern understanding of the hygiene hypothesis is that immune systems are trained by environmental microbes, not by contracting clinical illnesses. Pathogenic viral infections like the flu are purely detrimental, with no upside for long-term immune function. Getting sick is not a necessary 'training exercise'.

The fatigue, apathy, and loss of appetite you feel when sick are not just passive symptoms. They are an evolved, intelligent response to conserve energy by shutting down non-essential processes (like digestion and motivation) to redirect that energy budget to fight infection.

The American Academy of Pediatrics, operating on the precautionary principle, advised parents to delay introducing allergenic foods. This lack of early exposure prevented immune systems from developing tolerance, directly leading to a massive increase in food allergies and creating a disastrous feedback loop.

Contrary to the belief that enduring an infection "builds" the immune system, using appropriate antibiotics for bacterial infections is a modern miracle. The body is still exposed and mounts an immune response; the antibiotics simply assist in clearing the infection without impairing future immunity.