chemoprophylaxis
Chemoprophylaxis is the use of antimicrobial drugs to prevent an infection that has not yet happened, rather than to treat one already established. It is the medical equivalent of carrying an umbrella before the rain: the drug is on board and ready, so that if exposure occurs, the microbe cannot gain a foothold.
It is deployed in defined high-risk situations. Surgeons give a single dose of antibiotic just before an operation to prevent wound infection; travellers take antimalarials before, during and after visiting an endemic region; people exposed to meningococcal meningitis or to HIV are given short courses to head off infection; and dentists may give prophylaxis to patients at high risk of endocarditis. The aim is always to cover a specific, time-limited window of exposure.
The central tension of chemoprophylaxis is that it gives drugs to people who are not ill, so the benefit must clearly outweigh the costs of side effects, expense, and added selection pressure for resistance. For this reason genuine prophylaxis is targeted and time-limited, not open-ended, and indiscriminate preventive antibiotic use is precisely the kind of misuse that fuels resistance.
A traveller to a malaria-endemic region takes atovaquone-proguanil daily as chemoprophylaxis, starting before arrival and continuing after return.
Prophylaxis covers a defined exposure window rather than treating active disease.
Pre-exposure prophylaxis for HIV, known as PrEP, is a prominent modern example: at-risk people take antiviral drugs continuously to prevent infection before any exposure occurs.