The chain of infection

The chain explains something the route vocabulary alone cannot: a single route is not a single link. When you classify transmission as direct or indirect you are naming the middle link, but every named route in fact reaches across several links at once, and that is what makes a barrier choice a clinical decision rather than a memorised label. Take airborne tuberculosis. The agent is the tubercle bacillus, the reservoir is another infected person, the portal of exit is the respiratory tract, the route is the dried droplet nuclei the patient has already exhaled, the portal of entry is the lung of the next person, and the host link is decided by immunity, nutrition, crowding and HIV status. A nurse who reaches only for a surgical mask has touched the route and nothing else; a nurse who isolates the patient in a ventilated room acts on the route, the exit and the host at once. Move to faecal-oral cholera and the same six slots fill differently: exit is stool, route is contaminated water, entry is the mouth, and the host link depends on stomach acidity and nutrition. Here soap, latrine and chlorination attack the route, whereas vaccinating a whole camp attacks the host link. Blood-borne HIV shows the pattern again: exit and entry are both blood, the route is the shared needle or the unprotected sexual act, and the reservoir is another infected person. The barrier is therefore chosen twice over, once by the route and once by whichever end of the chain the nurse can actually reach. Ask which link your hands, your budget and your authority can close today, then ask which other links stay open behind it.

Herd immunity works because each immune person is a dead end for the agent. If enough people are immune, the average infected person passes the infection to fewer than one other person, and the outbreak shrinks instead of growing. The number of new infections that one case would generate in a completely susceptible population is called the basic reproduction number, written R₀. From R₀ we can calculate the proportion of the population that must be immune to stop sustained transmission: the herd immunity threshold is 1 − 1/R₀. Measles is one of the most transmissible diseases known, with an R₀ usually estimated between 12 and 18. That gives a threshold of roughly 92% to 95%, which is why the lecture example of 90% of children vaccinated, although reassuring, may not be sufficient to stop measles, and why outbreaks still occur in well-vaccinated communities. A vaccine also has an effectiveness below 100%, and measles control requires two doses, so the proportion actually protected is lower than the proportion vaccinated on paper. Immunity can wane with time, and some agents change their surface proteins by antigenic drift or shift, so a population that was protected last year may be partly susceptible this year. Herd immunity shields people who cannot be vaccinated — infants too young, pregnant women, and people with immunodeficiency or allergy — but it works only if those people live within a well-vaccinated community. The nursing role includes explaining this arithmetic honestly, keeping the vaccine cold chain intact so doses remain potent, and completing vaccination schedules rather than accepting a single dose.

Each link suggests specific, evidence-based actions. Against the agent, we use sterilisation in an autoclave for instruments, disinfection of surfaces and equipment, and antimicrobial treatment that reduces the number of organisms a patient releases. Against the reservoir, we treat infected people, isolate or cohort patients carrying resistant organisms, control the rodents and insects that harbour zoonoses, and keep animals away from water and food. Against the portal of exit, we use masks and tissues, safe handling and closed drainage systems, and we contain blood and body fluids at the point where they leave the patient. Against the mode of transmission, we practise hand hygiene, treat water by chlorination or boiling, pasteurise milk, cook food thoroughly, ventilate rooms, and apply standard and transmission-based precautions. Against the portal of entry, we protect skin and mucosa with aseptic technique, wound care, catheter care and eye protection, and we keep ports and lines closed. Against the susceptible host, we vaccinate, improve nutrition, treat chronic disease and shield the immunocompromised. The framework also shows why some measures are wasted effort in a given outbreak: scrubbing floors will not stop an airborne virus such as measles, and routine hand hygiene will not prevent rabies — once a bite has occurred, prompt thorough washing of the wound with soap and water, followed by vaccination and immunoglobulin, is part of post-exposure management rather than a link in the routine chain. Nursing assessment therefore asks which link is currently open, then chooses the barrier that closes that link. Documenting the choice — isolation, vaccination, or referral for water and sanitation — is part of the nurse’s duty and prevents busy work that breaks nothing.