Update — Jan 30, 2018: Alberta Health has provided a more thorough explanation as to why the rate of sexually transmitted infections is so high in southwest Calgary and northeast Edmonton, in particular. It has to do with how anonymous tests are recorded. Details have been added to the story below.

Rates of sexually transmitted infections (STIs) have been on the rise in Alberta and, despite a recent spike in gonorrhea and syphilis rates, chlamydia remains the most likely disease to catch if you're having unprotected sex.

But the distribution of chlamydia infection varies widely based on age and geography.

The following graphs show the demographic trends of chlamydia from 2010 to 2014 — the last five years from which Alberta Health has published full data sets.

Grande Prairie edges out Red Deer

Among Alberta's two biggest cities, Edmonton has historically had higher chlamydia rates than Calgary.

But factor in the province's other major cities, and you see a different pattern.

Red Deer used to have the highest rates but has since been outpaced by Fort McMurray and Grande Prairie.

Click on a year to change the data displayed by the interactive graph:

Highest rates in S.W. Calgary...

Within the Calgary and Edmonton health zones, the data breaks down even further, with different rates for different parts of each city.

Alberta Health bases the geographic information on the registered postal code of patients and, as you can see, southwest Calgary appears to have a significantly higher chlamydia rate than the rest of the city.

But there's a reason for that.

"If the person who's tested either doesn't have a fixed address or if they request anonymous testing, they're assigned a general delivery postal code for the city that they live in, and that general delivery postal code happens to be in that quadrant," explained acting deputy chief medical officer of health Deena Hinshaw.

"The bulk of the difference ... is really driven by the volume of tests that are assigned that general delivery postal code."

...and in N.E. Edmonton

Similarly in Edmonton, the northeast has historically seen the highest chlamydia rates, which Hinshaw said is largely related to the same issue with anonymous tests being assigned a general delivery address in that part of the city.

That disparity spiked in 2012, when northeast Edmonton had more than triple the rate of any other part of the city.

Chlamydia rates have since declined in the northeast, while rising in the rest of the city.

Peak rates at age 20 to 24

Perhaps not surprisingly, chlamydia rates are strongly correlated with age, with teens and young adults having the highest rates of any age group.

Albertans aged 20 to 24, in particular, have the highest rate, and that has grown each and every year for the past five years on record.

Other age groups have also seen increases, however.

Between 2010 and 2014, for example, the chlamydia rate grew by 58.6 per cent among 40-to-44-year-olds and and 70.5 per cent among 45-to-49-year-olds.

To combat the STI trend, Alberta Health Services plans to "step up its awareness and outreach efforts," according to medical officer of health Dr. Gerry Predy.

"The effects of untreated STIs can extend to other sexual partners or babies of infected pregnant women," Predy said. "Although many STIs can be treated and cured, if left untreated they can result in serious health impacts and long-term implications."

AHS has also expanded STI clinic hours in Edmonton and Calgary and will continue to promote its website — www.sexgerms.com — and related social-media campaigns to raise awareness and encourage Albertans to get tested.