South Africa has experienced the Covid-19 epidemic rather differently from many other countries and there is much interest and speculation why this might be.

It appears to be different in two ways. First, the total number of infected people presenting with illness or discovered through limited community testing is relatively low (4,220 as of April 25). Compared with the United Kingdom, or with developing countries, such as Brazil and Mexico, there are far fewer cases per million population, and South Africa finds itself in the company of countries such as Finland, Egypt and Argentina.

The first explanation for these lower rates is simply that the initial infectious cases appeared in South Africa later than they appeared in the UK and US. One way of marking