Why is it important to assume that the rate of radioactive decay has remained constant over time?

It remains constant because it is an element. Just like Chernobyl, there is still active radiation. Radiation is something that stays in the air for a vey long time. Radioactivity kill cells, so lets say you have your phone on but its in your pocket.the radiation from the phone is killing your cells.

The immune system fights off infectious disease. This is the system which produces cells that fight off the common cold. This system is also very closely related to the lymphatic system. Some people have much stronger immune systems that others.