How Does Chlorine Kill Fish?

Just curious...

How does chlorine affect fish? If you forgot to add your water
conditioner would your fish succumb to the toxins in minutes, hours,
days or just leave them susceptible to infection?

I know someone is going to say it depends on how much chlorine, but for
the sake of argument, let's just say "average" tap water treated with
chlorine. (Whatever that means)

Then, same question again but with chloramines.