Poor diet, pollution, stress.
Scientists have proved that the human immune system is at its strongest when we eat 30 or more different fruit & veg a week.
In the past, people grew their own veg, ate whatever was in season, ate a far higher percentage veg because meat was much more expensive, ate more fruit. Veg were fresh, not stored in Tesco warehouse for six months.
Now many people eat frozen peas & sweetcorn, maybe a bit of salad and the garlic & onion in their take away curry.
Hardly surprising, with a depleted immune system we can't fight off the more serious diseases.
We have vaccines now to protect against measles and tetanus, smallpox, diphtheria, whooping cough etc, the big killers of young people in the past, but there is no vaccine for many forms of cancer.
Medical science is getting there. We can protect against cervical cancer now, soon the NHS will dispense a preventative for some forms of breast cancer, but there's a long way to go.