To control infectious disease it is helpful to understand how organisms are disseminated and gain access to the pig. Each organism has individual properties that determine how long it will survive outside the pig, how infectious it is, and how easily it is transmitted. The methods by which disease spreads include the following:

• Direct contact with the infected pigs, including newly purchased pigs.
• Mechanical dissemination by vehicles, particularly pig transporters.
• Mechanically by on farm equipment, boots and clothing.
• Infection transmitted by people (e.g. influenza).
• Movement of birds, rats, mice, flies, dogs, cats and wildlife (e.g. wild boars).
• Environmental contamination on the pig farm (e.g. moving pigs into a contaminated pen, movement of contaminated faeces along defecating passages).
• Contaminated food and water.
• Airborne transmission in aerosol droplets or dirt.
• Biting insects (e.g. eperythrozoonosis).