Infection prevention and control rely on understanding how microorganisms move between people. The spread of disease follows a predictable cycle known as the Chain of Infection. This model illustrates the sequence of events necessary for an infection to occur, providing a framework for stopping the spread of illness. Identifying the weak points in this chain allows public health measures, such as handwashing, to interrupt the transmission cycle.
The Components of the Infection Chain
The Chain of Infection is composed of six distinct links, and all six links must remain unbroken for a disease to successfully spread. The cycle begins with the Infectious Agent, which is the pathogen—such as a bacterium, virus, fungus, or parasite—capable of causing disease. This agent then resides in the Reservoir, which is a place where the microorganism lives, grows, and multiplies, such as a person, an animal, or a contaminated surface or object.
The pathogen must then find a Portal of Exit, which is the route it uses to leave the reservoir, often through respiratory secretions, feces, or an open wound. Once outside, it must travel via a Mode of Transmission, which describes how the agent moves, such as through direct contact, airborne droplets, or contaminated hands and surfaces. This traveling pathogen then requires a Portal of Entry into a new individual, frequently through the eyes, nose, mouth, or a break in the skin. The final link in the chain is the Susceptible Host, which is the individual who lacks sufficient immunity to resist the invading pathogen. If any single link in this entire sequence is broken or eliminated, the infection cannot continue its journey to a new host.
How Handwashing Disrupts Transmission
Handwashing primarily functions by physically breaking the Mode of Transmission link in the chain of infection. The mechanical action of rubbing the hands together with soap and water generates friction that dislodges transient microorganisms from the skin’s surface. Soap molecules lift these pathogens and the oils that harbor them, allowing the running water to rinse them away completely.
This physical removal prevents the hands from acting as vehicles that carry pathogens from a contaminated source to a new person or surface. By eliminating the means of transport, hand hygiene directly interrupts the most common pathway for indirect contact spread. This practice also reduces the potential for hands to serve as a temporary Reservoir for pathogens picked up from the environment or from an infected person.
When hands are cleaned, the barrier to the Portal of Entry is simultaneously reinforced. People frequently touch their eyes, nose, and mouth—the body’s main entry points for many respiratory and gastrointestinal infections—without realizing it. Removing the pathogens from the hands ensures that even if a person touches their face, the infectious dose is significantly reduced or eliminated, protecting the susceptible host.
Critical Moments for Hand Hygiene
The effectiveness of hand hygiene relies heavily on situational awareness and proper timing to ensure the chain is broken before transmission can occur. A critical time for intervention is before engaging in activities that carry a high risk of transmitting pathogens to the face or mouth. This includes always washing hands before preparing food or eating a meal.
Hand hygiene is necessary immediately after situations where contamination is likely, such as after coughing, sneezing, or blowing the nose. These actions release respiratory droplets that can contaminate the hands, which then transfer to surfaces. Hands must also be cleaned immediately after using the restroom.
Other important moments involve interacting with potentially contaminated environments, like after touching a pet, handling pet food, or taking out the garbage. In a caregiving context, hands should be cleaned both before and after caring for someone who is sick, to protect both the caregiver and the patient’s environment from cross-contamination.