Question

one use of radioactivity and harmful effect

Answer 1

use of radioactivity- 
1) Many diseases such as cancer , leukemia are  treated with radio therapy. for e- Gamma radiations obtained from cobalt-60 is used for treating cancer.
on harmful effect - 
1) It result in short term recoverable effects such as dirrahea and can have genetic effects.

Answer 2

Disadvantages of radioactive

If this happens in our body, the cells may die or they may undergo a change called a mutation. The result is called radiation sickness. A large dose of radiation will cause death!
advantage of radioactive
Killing Microbes:

Gamma rays successfully kill microbes that cause food to decay. So food treated with this radiation have a longer shelf life. Surgical instruments and syringes are also treated with gamma rays, in order, to prevent infections been transferred from patient to patient.