because they want to carry out the day activities without undue fatigue.
is not allowed
2 and 4
Having fatigue doesn't mean you have a STD.
Je suis fatigue aujourd'hui mean "I am tired today"
undue reliance
No, "undue" is not a preposition. It is an adjective used to describe something that is excessive or unwarranted.
A narrow-minded person who pays undue attention to formal rules
Chronic fatigue syndrome
Fatigue/malaise
Undue
general weakness, fatigue