Essential is related to Requirement. Requirement is related to Believe. Believe is a different concept from Doubt. Doubt is a synonym of Hesitate. 
Virus is a type of Species. Species is a type of Kind. Kind is related to Test. Test is related to Demonstration. 
Essential <--RelatedTo--> Requirement <--RelatedTo--> Believe <--DistinctFrom--> Doubt <--Synonym--> Hesitate 
Virus --IsA--> Species --IsA--> Kind <--RelatedTo--> Test <--RelatedTo--> Demonstration 