It infected some of the people with fever (6)
I believe the answer is:
plague
This clue could be a double definition.
'it infected some of the people' is the first definition.
The definition suggests a singular noun which matches the answer.
'fever' is the second definition.
(I have seen 'Contagious fever ' mean 'plague' so perhaps 'fever' could also mean 'plague')
'with' acts as a link.
Can you help me to learn more?
(Other definitions for plague that I've seen before include "Contagious fever" , "Pester with deadly disease" , "Large-scale illness" , "Deadly epidemic" , "Contagious disease that spreads rapidly and kills many" .)