Charts updated April 20, 8 a.m. ET.

The new coronavirus has been spreading quickly in Canada.

How many cases are really out there? It’s impossible to tell since not everybody who may have the virus gets tested and not all cases show symptoms.

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Here are the known cases so far.

When you look at their share of the population, Canadians between 50 and 70 appear to be more likely to have been diagnosed with the disease and those under 19 much less likely.

(In Italy, the elderly have made up the overwhelming majority of COVID-19 deaths.)

There are two cautions with the data, though. One is that we have age data for roughly half of the Canadians with a COVID-19 diagnosis. The other is that many people with no reason to seek out medical attention turn out to be carrying the virus. In Iceland, which is testing on an ambitious scale, about half of people with positive tests showed no symptoms.

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Here is an age breakdown of cases for which we know the age, along with their proportion of the overall population:

Here is a graph of high-population provinces.

In recent days, Quebec has had Canada’s highest rate of positive tests.

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Here’s another look at the same data, this time expressed as a rate per 100,000. Quebec still has far more cases in relation to its population, but Ontario, B.C. and Alberta look much more similar to each other.

Here’s what it looks like if you adjust for population differences:

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Something else to bear in mind, though, is that provinces and territories test at very different rates. As Iceland found, the more you test, the more you find.

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Something else to keep an eye on is the percentage of tests that come back positive.

The World Health Organization said this week that places where most tests come back positive are doing a poor job of keeping track of the virus, and probably have many cases out there that aren’t being found. The WHO likes to see positive rates around 10-12 per cent, which it sees as a sign of vigilance.

The good news is that every Canadian jurisdiction is doing better than that.

However, a look at the positive rates of tests conducted shows where the stresses on the testing system are. Newfoundland and Labrador and Quebec lead the country, followed some distance back by Ontario. Ontario has increased its testing in recent days.

Further down the list are the Western provinces, followed by the territories, which have been testing much more aggressively than communities in the south.

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(We have no data for Nunavut.)