This Canada Day weekend, it's not a good idea to don swimwear and head to Sunset Beach in Vancouver's West End, no matter how good the weather might be.

That's because E. coli counts have been recorded at double the allowable amount for a single sample.

A geometric mean of less than 200 E. coli per 100 millilitres is required over five previous samples to shut down swimming.

Single samples over 400 E. coli per 100 millilitres can also trigger an advisory. 

The most probable number at Sunset Beach was 798 per 100 millilitres in a single sample, according to Vancouver Coastal Health.

The regional health authority posts signs with a "no swimming" advisory when there is an increased risk of illness to swimmers.

There's also a swimming ban at Snug Cove off Bowen Island because the geometric mean was recorded at 256 per 100 millilitres.

Seniors, infants, children, and people with reduced immunity are most vulnerable when E. coli counts rise.

"The risk of getting sick is higher if you swallow water or get water in the nose, eyes, ears, or an open wound," Vancouver Coastal Health states on its website. "Examples of possible illness include stomach upset, ear infection, sore throat, or wound infection."

There are various sources for E. coli contamination, including sewer overflows, discharge from boats, animal waste, and storm runoff with contaminants from other sources, including recreational vehicles.