What makes them so strong and deadly?
Blast wave effects on humans
Radiation effects on parts of the body
Brain -- The brain will not be damaged unless the radiation is 5,000 rems or higher. This could result in seizures or immediate death.
Thyroid -- In plentiful amounts, radioactive iodine can destroy parts of the thyroid or all of it
Blood system -- If the person is exposed to 100 rems the blood's lymphocyte count will decrease. This would make the victim much more susceptible to infection.
Heart -- At 1,000-5,000 rems of radioactivity would immediately damage blood vessels and could cause heart failure or death