What must occur for disease transmission to happen?

For disease transmission to occur, multiple criteria need to be met. A susceptible person must exist, as individuals with weakened immune systems or those who have not been exposed to the disease before are more likely to catch it. Additionally, a pathogen must be present; without the organism that causes the illness—such as a bacteria, virus, or fungus—transmission cannot happen. The process of disease transmission involves a chain of events, where all these components interact to facilitate the spread of the disease. If any one of these necessary conditions is missing, transmission is unlikely to occur. Therefore, stating that all of these conditions must be met accurately reflects the complexity of how diseases spread from one host to another.