Because chronic illnesses begin to appear during middle adulthood, the death rate during this period is stable is about the same decreases increases

The answer to this question would be: increases

When the medical field is not developed yet, the lifespan of the human is much lower than today. In this case, the human that survive adulthood is much less so there are fewer people with chronic illness. This will increase the number of chronic illness as the people who prone to the disease is increased too.