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The Middlesex-London Health Unit has issued an alert about a continuing outbreak of invasive group A streptococcus (iGAS).

There have been 132 reported cases of the bacterial infection since April, 2016.

To date, there have been nine deaths, 20 cases of necrotizing fasciitis (flesh-eating disease), 20 cases of toxic shock and 29 others have ended up in intensive care with other grave symptoms.

Those numbers are frightening, but they are just a small part of a much larger story.

In recent months, there have been similar outbreaks of this particularly nasty strain of strep A in Toronto, Montreal, Vancouver, Calgary and likely elsewhere.

In every city, there have been deaths and a number of amputations caused by flesh-eating disease, but the story has stayed under the radar because this type of infection largely affects intravenous-drug users and the precariously-housed, especially those living in shelters.

Disease and death stalk marginalized communities quite routinely, so outbreaks of infectious diseases like iGAS don't generate much attention from public-health officials and the media.

That is true in London as much as anywhere else.

But what happened in London is that strep A started harming people in the general population as well.

"There has also been an increase in iGAS infections among people who have no connection, and are not related, to the outbreak in people who inject drugs or who are under-housed. We need a better understanding of what's happening, which is why we've issued this alert," Dr. Gayane Hovhannisyan, the associate medical officer of health, said in a statement.

In many ways, this is maddening.

Why don't we care about infectious diseases until they start to affect school children, university students and the suit-and-tie wearing members of society?

The reality is that almost every infectious-disease outbreak – from annual influenza through to pandemics like AIDS – strikes the vulnerable first. That's how it gains a foothold to wend its way into the general population.

In other words, there is a great deal of self-interest in doing better health surveillance, and providing better health care, to members of marginalized communities such as the homeless, drug users and sex workers.

But we do the exact opposite.

Overcrowded shelters, for example, create the ideal conditions for the spread of disease. That's why the outbreaks of iGAS were concentrated at the Salvation Army Centre for Hope in London, Seaton House in Toronto and Old Brewery Mission in Montreal.

That being said, the Middlesex-London Health Unit is right to further explore how invasive strep A has spread into the general population.

Bacterial infections like strep A are commonplace; an estimated 1 in 10 children and 1 in 100 adults have strep A bacteria in their throat or on their skin at any given time. For the most part, it's innocuous, causing mild symptoms.

But when strep A gets into tissue or bone – usually because a person has an open wound – it can turn deadly, and quickly.

In a matter of hours, an infected person can go from having a mild fever to having flesh-eating bacteria spreading so relentlessly that the only way to stop the infection is by amputating limbs.

Thankfully, this is a relatively rare phenomenon. In Canada, only about 100 people a year die of invasive strep A infection, and they tend to be immuno-compromised, either because of underlying illness such as cancer or poverty-related poor health.

There is also some bad luck involved. Muppets creator Jim Henson died of toxic shock caused by strep infection; then-Bloc Québecois leader Lucien Bouchard had a leg amputated as a result of flesh-eating disease caused by strep A.

Non-invasive strep A also causes common illnesses such as strep throat, impetigo and scarlet fever.

What is highly unusual is clusters of severe disease among seemingly healthy people like we're seeing in London.

The nature of the outbreak hints at a couple of things, like intravenous-drug use and precarious housing being far more common phenomenon in the general population than we care to admit.

The way these various outbreaks are being treated in isolation is also a sad reminder of what a terrible job we do at national tracking of infectious disease.

In addition to collecting data, the Public Health Agency of Canada needs to do a better job of figuring out how the pieces of the puzzle fit together, because the alert could be relevant well beyond London.