This finding is particularly relevant for diseases such as Influenza or Measles, which spread via respiratory droplets that can be transmitted from person to person through coughs and sneezes. Higher densities of people taking public transportation increase the likelihood of these indirect interpersonal contacts, which facilities the formation of large scale epidemics.
The paper also offers suggestions for interventions that could help curb this so called "transportation effect," taking into consideration the cost effectiveness of the measures. Adda states that school closures and shut downs of transportation networks in the midst of large scale epidemics are helpful, but only cost effective if the virus is "more deadly that the typical flu, as in the 2009 swine flu outbreak." He adds that school closures in combination with longer vacations in the winter (when most viruses spread) and consequently shorter breaks in the summer (when fewer viruses are "in season"), are a good start to finding better solutions.
As countries across the globe become more developed, the idea that economic prosperity may also contribute to the burden of viral infections is daunting, yet this reinforces the importance of vaccination and prevention. If more people are vaccinated against a disease, the less likely it is that it will spread; and we won't have to fear that economic progress is something not to strive for.
Link to article: https://www.sciencedaily.com/releases/2016/10/161004085307.htm
Link to paper: http://qje.oxfordjournals.org/content/131/2/891.full.pdf+html