Ro

**Glenn Webb, PhD**

Vanderbilt University, Nashville, Tennessee, United States

**Poet’s statement:**In epidemiology,

**R**(pronounced R-naught) is the measure of an epidemic. It is defined as the mean number of secondary cases produced by a typical single infected case in a population of susceptible individuals.

_{0}**R**is an indicator for the severity of the epidemic as it evolves and is obtained from various parameters that determine the dynamics of the infection. If

_{0}**R**< 1, then the epidemic will abate. If

_{0}**R**> 1, then the epidemic will worsen. The higher the value of

_{0}**R**, the more severe the epidemic will be. At its peak, the

_{0}**R**for the 1918 influenza pandemic was estimated to be between 2.0 and 3.0.

_{0 }The graph of **R _{0}** for a hypothetical disease has a function of two input variables: (1) the percent of the susceptible population quarantined and (2) the number of days an infected person is asymptomatic while infectious. The horizontal blue plane is at the critical value 1. The values of

**R**on the red surface are higher for lower percentages of susceptibles quarantined and for longer periods of asymptomatic infectiousness.

_{0}**R _{0}**

Poor R

_{0}, so much abused.

Its calculation confused.

Sometimes high, sometimes low, sometimes blamed

For being wrong.

What it is ‘tis hard to say,

Parameters change day by day.

All it wants is some respect,

To be correct.

Microbes try to raise it up.

Doctors try to make it fall.

All the while mathematicians

Try to figure its position.

When high, it plays a tragic role,

On countless souls it takes its toll.

When plague appears, we fear

Its unknown.

For each new scourge it tells our fate

With factored in transmission rate.

Get down R_{0}! Know your place! Do not torment

The human race!

**GLENN WEBB,** PhD is a professor of mathematics at Vanderbilt University in Nashville, Tennessee. Much of his research involves mathematical models in medicine, including models of tumor growth, cancer therapy, antibiotic resistance in hospitals, and epidemics such as HIV, anthrax, influenza, and prion disease.