Viral Coefficient (K-factor)

Biological Parallel

The basic reproduction number (R₀) in epidemiology is biology's viral coefficient: how many new infections each infected individual generates. When R₀ > 1, exponential spread occurs; when R₀ < 1, the outbreak dies. COVID-19's R₀ of 2-3 drove pandemic growth; measles' R₀ of 15 makes it explosively viral. Startups chase the same threshold: if each user brings in more than one additional user (K > 1), growth becomes self-sustaining. Below that threshold, growth requires continuous external fuel.