The basic reproductive ratio, R
0, is one of the fundamental concepts in mathematical
biology. It is a threshold parameter, intended to quantify the spread
of disease by estimating the average number of secondary infections in a wholly
susceptible population, giving an indication of the invasion strength of an epidemic: if
R
0 < 1, the disease dies out, whereas if R
0 > 1, the disease persists. R
0
has
been widely used as a measure of disease strength to estimate the effectiveness
of control measures and to form the backbone of disease-management policy.
However, in almost every aspect that matters, R
0
is flawed. Diseases can persist
with R
0 < 1, while diseases with R
0 > 1
can die out. We show that the same
model of malaria gives many different values of R
0, depending on the method
used, with the sole common property that they have a threshold at 1. We also
survey estimated values of R
0
for a variety of diseases, and examine some of the alternatives that
have been proposed. If R
0
is to be used, it must be accompanied by caveats
about the method of calculation, underlying model assumptions and evidence
that it is actually a threshold. Otherwise, the concept is meaningless.