For example, if a country of 4.8 million people tests 12000 people a day, then, assuming that testing is evenly distributed across the whole population, every person will be tested, on average, once every 400 days.<p>To fully control the spread of disease, the testing rate probably has to be no more than a small multiple of the average incubation period of 5 days, ie 10-20 days.<p>If we adopt "days" as a standard way of describing testing rates, then it provides a number which everyone can easily compare to the incubation period of the disease.