Direct Transmission

"A new infection begins when pathogens leave the body of their host—the infected individual in which the pathogens are multiplying—and enter a new host. They may be repelled by defense mechanisms, such as skin, in the new host, or they may survive and reproduce in sufficient numbers to cause an infectious disease. Transmission of pathogens can occur directly between people, or indirectly in the air, water or food, via other animals to humans, or from sources in the environment. Direct person-to-person transmission includes contagious infection, when touch, such as a handshake, transfers pathogens to a susceptible person; sexually transmitted infection, involving infected semen, vaginal secretions, saliva or blood during unprotected sex; and mother-to-child transmission, when pathogens pass from mother to baby during labor and delivery or via breast milk. Sexual transmission is the most common route for the worldwide spread of HIV, which causes AIDS. Sexual transmission is more likely if the partner’s genitals, mouth or rectum are inflamed, for example, by another STI such as gonorrhea or syphilis."