Why are most vascular pathogens poorly communicable from person to person?

Most vascular pathogens are poorly transmissible from person to person because they require direct contact with the blood or secretions of an ill person to be transmitted.

Explanation:

Vascular pathogens - more commonly called bloodborne pathogens - are those found in human blood or secretions and are capable of producing specific diseases, such as human immunodeficiency virus, dengue fever or hepatitis B.

Unlike influenza, which is airborne and easily spread from person to person, bloodborne pathogens can only be transmitted through contact with contaminated blood or secretions:

During sexual intercourse. Through kissing, through exchange of saliva. Through blood transfusions. Workplace accidents of health care personnel, through cuts with contaminated instruments. From mother to child during pregnancy or birth. Through vectors - such as mosquitoes or other insects - that have ingested contaminated blood and transmit it to healthy people with the bite.

This makes the transmission of blood or vascular diseases less than other contagious diseases.