Hello and thanks for downloading the more or less podcast, with a program that looks at the numbers known, the numbers unknown and a known unknown number of numbers and I'm Tim Harford.

On the 17th of May, the World Health Organization declared that a new outbreak of the Ebola

virus in the Democratic Republic of the Congo was an international emergency.

Ebola virus is an extremely nasty viral disease with a high death rate.

But despite the severity, little is known about the number of infections in this current outbreak,

in part because this particular species of Ebola is a rare one.

Loyal listener Richard Ellis got in touch after reading a BBC news article that said,

modelling by the MRC Centre for Global Infectious Disease Analysis

suggested there had been a significant under-detection

of Ebola cases. They stated the true number could be over 1,000. Richard was curious about

how they came to these numbers. I'd love to understand more about how this kind of disease

modelling works. I mean, how do researchers estimate the number

of unreported cases and how reliable are these models likely to be in practice?

I'm also curious about what it means if the true number of cases is already much higher than reported.

To answer Richers' questions, we spoke to Dr Ruth McCabe.

She's an infectious disease epidemiologist at Imperial College London,

and she worked on the modelling behind those estimates.

Let's start with what we know.

Ebola is an incredibly serious disease.

It's what we would call a viral hemorrhagic fever. It typically has

quite a high mortality and it spreads between contact with bodily fluids from an infected person.

Bodily fluids, including sweat, saliva and blood.

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