This is the 12343rd most frequent Swedish word.
sjukdomen
"The disease" or "the illness."
The word 'sjukdomen' (the disease) is used here as a subject referring to an illness spreading in a specific context.
Sjukdomen har spridit sig snabbt i området.
The disease has spread quickly in the area.
Here, 'sjukdomen' is the direct object receiving the action of being researched against.
De forskar intensivt för att bekämpa sjukdomen.
They are conducting intensive research to combat the disease.
In this sentence, 'sjukdomen' appears as the object in a clause stating the importance of understanding it.