Prior to the second half of the 20th century (curve A), international transportation systems were remarkably slow since they were limited to sailship or steamship speeds. Even with rail, North America could not be crossed in less than a week. In such a setting, a pandemic could emerge in a matter of months. The setting of modern transportation systems (curve B), particularly air transportation, had two significant impacts on pandemics:
- Velocity. Transportation reduces the time at which a pandemic has run half its course; T(mid). Additionally, the diffusion rate is accelerated implying that a large population can be infected in a lesser amount of time. The pandemic diffusion curve, therefore, shifts from A to B.
- Extent. Because transportation is almost ubiquitous, very few regions of the world do not have close access to air services, a larger population can be impacted; P(max).
Another important aspect is the issue of critical time, T(C), which is when the translocation phase is beginning. It implies that after a certain amount of time the potential lethality of disease is realized and governments and individuals start to react with a variety of mitigating measures (quarantines, travel restrictions, institutional closures, absenteeism, etc.), which aim at lessening the growth rate of new infections. Due to its velocity, transportation has the potential to diffuse a pandemic to a large geographical area and an extensive population before its true nature is realized. At this point, it could be essentially too late to prevent the diffusion of a pandemic.