Is it ethical to deny people health insurance?

Nowadays, it seems that many people do not have health insurance, not only are many people not offered health insurance, but many people are also denied health insurance. People who have serious and terminal health conditions are often denied insurance, mainly because they will cost the insurer money. Does not this totally defeat the purpose of health insurance? Why have health insurance if you are not supposed to use it? It seems unethical to deny someone health insurance because they might actually use it. I think that it is wrong to deny someone health insurance because they have a health condition. Many people who have health problems did not choose to have them, and they would rather not have a health condition. I think there should be a better system of offering people who have health problems insurance coverage.