you are likely to find Sex in/at/on Bedroom. Bed is part of Bedroom. you are likely to find Snake in/at/on Bed. Snake is related to Animal. 
Fatigue is a type of Illness. Infection is a type of Illness. Infection is related to Disease. Disease is related to Transmission. 
Sex --AtLocation--> Bedroom <--PartOf-- Bed <--AtLocation-- Snake <--RelatedTo--> Animal 
Fatigue --IsA--> Illness <--IsA-- Infection <--RelatedTo--> Disease <--RelatedTo--> Transmission 