Smallpox is highly contagious disease. People get infected with smallpox virus by inhaling droplets of saliva, which are full of virus, during face-to-face contact with infected person.
Diphtheria is spread (transmitted) from person to person, usually through respiratory droplets, from coughing or sneezing.
Influenza viruses are spread mainly by droplets made when people with flu cough, sneeze or talk.