Answer:
The pathogens that are transmitted by contact mechanism from one person to another person is less virulent than those pathogens which are transmitted by arthropod vectors because arthropod vectors provide more inoculation of the pathogen in the body of the host.
Amount of inoculate of pathogen is important to cause a disease so in arthropod vectors the pathogen grow, mature and increase there number.
Therefore during transmission the number of pathogen is enough to cause a virulent disease but in contact transmission only small amount of pathogen gets in contact with the host therefore the virulence of these disease are less.