Nakkuslikkus

Nakkuslikkus is Estonian for infectiousness, the capacity of a pathogen to cause infection in a susceptible host and to be transmitted from one person to another. In epidemiology it is a component of transmissibility, alongside factors such as virulence and the ease with which transmission occurs. Nakkuslikkus describes how readily an infected individual can spread a pathogen to others, not just whether the pathogen can cause disease.

Determinants of nakkuslikkus include pathogen characteristics (for example, how easily it replicates, how much it is shed, and how stable it remains outside a host), host factors (immunity, age, behaviour, symptoms like coughing or sneezing), and environmental conditions (crowding, ventilation, hygiene). Transmission routes—airborne, droplet, contact, fecal-oral, vector-borne—also shape infectiousness.

Measurement and concepts commonly used in public health involve the basic reproduction number (R0) and the effective reproduction number (Rt), generation time, serial interval, and secondary attack rate. Infectiousness often varies over the course of infection, and some pathogens can be transmitted before symptoms appear or from asymptomatic individuals.

Public health relevance includes how interventions reduce nakkuslikkus. Vaccination lowers susceptibility and can reduce transmission; isolation and contact tracing limit spread; nonpharmaceutical measures such as masks, ventilation, and hygiene also diminish infectiousness.

Examples of highly infectious diseases include measles, which is known for its strong contagiousness, while influenza and SARS-CoV-2 show substantial infectiousness that varies with time and viral characteristics.