Why do patients need to undergo a series of tests to diagnose their illness?

검은색:학생영작=> [Student's sentence]
파란색:수정문장=> [Corrected sentence]
빨간색:문법설명=> [Grammar explanation] 
초록색:나은문장=> [Better sentence]

I think Doctor and nurses can get a clear diagnosis of diseases and determine how serious illnesses.
I think doctors and nurses can get a clear diagnosis of diseases and determine how serious illnesses are.
Professions are written in small letters.
Make sure you write in complete sentences

through series of tests can detect disease or prevent disease. 
Through a series of tests, they can detect diseases or prevent them.
Capitalize the first letter of a word in a sentence.
Make sure you write in complete sentences
Avoid repeating the same word repeatedly; use pronouns instead.