The most common portal of entry for pathogens is the respiratory tract. The pathogens enter through dust particles or droplets that enter the respiratory tract. Some pathogens that enter this way are the viruses known as influenza, measles and varicella-zoster, which causes chickenpox.
The average adult can breathe in approximately 21 cubic feet of air every day, providing many opportunities for infection. Viruses infect the respiratory tract by attaching themselves to cells and multiplying. Some of the most common infections of this area are the common cold, bronchiolitis and pneumonia.
Other portals include the skin, gastrointestinal tract and genitourinary tract. Examples of infections include mumps, tetanus, malaria and AIDS.