Practical is a synonym of Virtual. Virtual is the opposite of Legal. Legal is related to Limit. Limit is related to Regulation. 
Fatigue is a type of Illness. Fatigue is a type of Symptom. Chill is a type of Symptom. 
Practical <--Synonym--> Virtual <--Antonym--> Legal <--RelatedTo--> Limit <--RelatedTo--> Regulation 
Illness <--IsA-- Fatigue --IsA--> Symptom <--IsA-- Chill 