Francesca Tomasi received her B.A. from the University of Chicago and currently does tuberculosis drug discovery research.
Whenever an infectious disease pops up somewhere, one of the first things epidemiologists want to know is how contagious the pathogen is. Understanding how quickly an illness may spread is essential in both deducing whether a public health intervention is warranted and coming up with a strategy. Mathematically, contagiousness is described by a number called R0, which is pronounced “R naught.” R0 tells you the average number of susceptible individuals who will contract an infection from one contagious person. Susceptible individuals are those who have not been infected by this agent and who have not been vaccinated against it.
There are three outcomes that stem from knowing a pathogen’s R0, each of which carries its own implications for whether an epidemic, a widespread occurrence of disease, will occur within a population.
For infectious diseases with an R0 greater than 1, appropriate measures need to be taken to curb a brewing outbreak. Suffice to say the larger the value of R0, the more difficult a pathogen is to control.
What goes into calculating a pathogen’s R0? The three main factors are infectious period, contact rate, and mode of transmission. The longer someone is contagious, the higher the chances that person will spread their illness to more people. Similarly, the more frequently a contagious person interacts with susceptible individuals, the more people are likely to get infected. This plays into visible versus invisible illness too: if someone is visibly ill, they are more likely to stay at home (or in a hospital) and avoid contact with the outside world. Furthermore, if somebody is diagnosed with an infectious disease, they may even be quarantined, a practice that dates back thousands of years, before people even knew what caused communicable diseases. The term “quarantine” itself comes from the Venetian dialect of the Italian phrase quaranta giorni, which means forty days. During the Black Death in the 14th century, ships were isolated for 40 days before their passengers could enter cities. Lastly, how a disease spreads will dictate how contagious it is. For instance, something that spreads through the air – like the flu – does not necessarily require physical contact and thus spreads quickly and easily. On the other hand, diseases like HIV that are transmitted by bodily fluids, are harder to catch and spread.
Let’s look at some common infectious diseases and their R0 values:
Just as with any tool, R0 carries its own limitations. In the event of vector-borne diseases like malaria, it is difficult to predict frequency of mosquito bites and boil it down to an average. Furthermore, R0 is often attributed as a threshold rather than an absolute quantity. Regardless, it is helpful to understand disease dynamics when informing public health interventions in the event of an outbreak.