What is meant by the chain of infection is a communicable disease from one host, which is a person, to another. It is meant that the chain of infection can be broken or the risk is reduced by breaking the chain of the infectious agent. The chain of infection is the way information is gathered to combat or stop an epidemic. The first link in the chain is the organism or infectious agent which is a bacteria or virus etc. The second link is the reservoir is where the infection is holding or reproducing such as humans, animals, doorknobs, etc. The third link is the portal of exit such as the nose, mouth, feces etc. The fourth link is the mode of transmission such as hands. The fifth link is the portal of entry like the nose, skin, mouth or contaminated food. The sixth link is vulnerable populations. These are individuals young and old who have low immune systems due to genetics, transplants, malnutrition or viral infections.
Breaking one of those links increases the chance of stopping the epidemic. However, it depends on which organism to focus on that will be most effective in breaking the chain.