1. Down the respiratory tract into the lungs. Coughs, cold, influenza and other common airborne infections are contracted in this fashion.
2. Breaks in the skin. One of the many functions of the skin is to act as a barrier against infection. Anything that penetrates the skin, or for that matter the mucous membrane that lines the mouth or nose, provides a route for infection to enter. Typically, bites, scratches, puncture wounds by needles etc increase the risk of infection.
3. Down the digestive tract. Food, drink or other infected products can be swallowed and infect the stomach or bowels. Most people have experienced an 'upset stomach', which reveals itself in the form of diarrhea and or vomiting.
4. Up the urinary and reproductive systems. The infectious agent may remain localized or may enter the blood stream. Sexually transmitted diseases most commonly infect the genitals. HIV, the AIDS virus, is carried in bodily fluids and can be transmitted in saliva, seminal fluid, or blood.