You get sick because:
1. No one is perfect. Humans are full of clumsy design defects. Our back muscles are too weak, so even normal activity can injure them. Human veins are too flabby to resist a normal blood pressure; that’s why we get hemorrhoids and varicose veins. Roughage may be good for you, but humans can’t digest some of its carbohydrates. That’s why a high fiber diet causes gas.
2. Your “resistance” is too strong. Don’t blame a fever on that infection you caught. Your body itself generates extra heat whenever an invader is present. Some experts claim fever is good for you, but the evidence is thin.
The human immune system is too strong for its own good - and trigger happy! Allergies occur when it tries to defend you against an innocent pollen, drug, or food. Even worse, it may decide one of your own organs is the enemy and mount a vicious attack. Thyroiditis, the most common cause of thyroid deficiency, is an “autoimmune” disease. An overactive immune system may also assault the heart (rheumatic fever), liver (chronic hepatitis), kidney (nephritis), joints (rheumatoid arthritis), or skin (eczema, hives).
3. Nature plays no favorites. It assumes germs and viruses have as much right to exist as you. Catching the flu, for example, is not a sign of weakness. Most healthy people exposed to a new flu virus get sick. They recover, but this is not a defeat for the virus. Multiplying during the illness, many escape into the air to infect others. The flu virus normally infects a victim for a week or so, then moves on. That’s the role it plays in nature. Your role is to get the flu now and then.
Medical science works to give humans an advantage in this competition, and we’ve done pretty well. But nature is impartial.