He fell sick a week before the exam and was diagnosed with acute malaria and typhoid.
A KCSE candidate sent home to seek treatment died on Saturday night after a short illness.
Basil Omondi developed malaria and typhoid in school and was allowed to seek medical care, his father Francis Ogama said on Sunday.
He spoke at the family home in Kadika Village, Migori County. The student schooled at Migori Complex Secondary School.
Mr Ogama said his 18-year-old son was rushed to Oruba Nursing Home and was diagnosed with acute malaria and typhoid. He was admitted in the hospital last week for three days before being discharged.
He added that a few days later, his condition worsened and he was again rushed to St Joseph Hospital Ombo last Friday evening and was given the same diagnosis.
The student was admitted but died on Saturday while undergoing treatment.
His mother Grace Akinyi said that despite his condition getting worse, Omondi wanted to go back to school for exam rehearsals last Friday morning.
She described her son as an active person who took part in many co-curriculum activities in the school – he was a member of the school drama club, music and handball team.
School Principal Edwin Agere said Omondi had complained of severe headache and muscle pains.
He said the examination supervisors had even asked to make arrangements to take the exam papers to the hospital so that the student sits for his tests, but he died before the written exams began.
The school has registered 66 students to sit for the form four exams that begin on Monday morning.
Omondi’s classmates received the news with shock and grief.
Their teachers counselled them as they prepared for Monday’s exam.
"The other candidates are evidently psychologically affected by the demise of their classmate but we are counselling them," said Mr Moses Nyaliech, Omondi’s class teacher.