Scientists at Columbia University’s School of Public Health recently led a study using computer models to track how major outbreaks move. They compared the spread of the 2009 H1N1 flu pandemic with the 2020 COVID-19 pandemic across major cities in the United States. The research showed that both viruses reached most parts of the country within just a few weeks, often circulating before doctors even noticed them. The study found that air travel was the main reason these diseases moved so quickly from city to city.