A science journalist writes that a virus spreads such that each infected person infects 1.8 others every 3 days. Starting with 5 cases, how many total infections occur by day 9 (assume geometric progression)?

Thu, 30 Apr 2026 18:09:53 +0000

Title: Tracking Virus Spread: How 1.8 Reproduction Rate Affects Infections Over Days—A Science Breakthrough Understanding how viruses spread is critical in predicting outbreaks and guiding public health responses. A recent analysis by a science journalist reveals fascinating insights into a virus with a reproduction rate of 1.8, meaning each infected person transmits the virus to 1.8 others every 3 days. Starting with just 5 initial cases, how many total infections occur by day 9? Using a geometric progression model, we uncover the power of exponential spread in real-world scenarios. ### What Does a 1.8 R0 Mean Genuinely? The R₀ (basic reproduction number) of 1.8 implies that, on average, each infected person infects 1.8 new individuals during a 3-day cycle. Unlike static infections, this transmission rate fuels rapid, compound growth in cumulative case numbers. This is especially relevant in metropolitan areas or dense social networks where contact rates remain high.