Ideas in this flowchart

• Pathogen transmission and the development of infection
• A new infection begins when pathogens leave one host and enter a new host.
• The original host is the infected individual in which pathogens multiply.
• What happens when pathogens enter the new host?
• The pathogens may be repelled by defenses such as the skin.
• The pathogens may survive and reproduce in sufficient numbers to cause infectious disease.
• Pathogens can be transmitted directly or indirectly.
• Direct transmission occurs between people.
• Indirect transmission occurs through air, water, food, animals, or environmental sources.
• Contagious infection transfers pathogens through touch, such as a handshake, to a susceptible person.
• Sexually transmitted infection involves infected semen, vaginal secretions, saliva, or blood during unprotected sex.
• Mother-to-child transmission occurs during labor and delivery or through breast milk.
• Indirect transmission can occur through the air, water, or food.
• Other animals can transmit pathogens to humans.
• Pathogens can originate from sources in the environment.
• Sexual transmission is the most common worldwide route for HIV, which causes AIDS.
• Sexual transmission is more likely when the partner’s genitals, mouth, or rectum are inflamed.
• Inflammation may be caused by another STI, such as gonorrhea or syphilis.