Two of the many things that scientists study are germs and bacteria that carry diseases; hence the need for protective clothing.
Many of these germs and diseases could be extremely dangerous if touched or sometimes just breathing in.
It is essential for the scientist to wear protective clothing which would include masks, gloves, body and head protection.
Scientists sometimes have to handle radioactive material.
Exposure to radioactive radiation can be fatal to any living organism. For this reason robots are used rather than human beings if at all possible.
When people do need to handle such substances they wear protective and carry a meter that records the amount of exposure to the radiation they are receiving.