Monday, April 21, 2008. The 112th running of the Boston Marathon. Helena Carvalho was two-thirds of the way to a three-hour-and-49-minute finish at one of the world's premier sporting events.

But with every graceful stride towards that finish, something nasty was brewing beneath the athletic sock on her left foot. A great big, bulging purple blister.

"I could feel it starting. And then the stickiness, I thought, 'oh, it's seeping out a little bit, there's going to be a big stain on my sock.'"

Helena Carvalho felt this blister forming around 30 km into the Boston Marathon on Apr. 21, 2008. (Submitted by Bryan Mulligan)

Despite the pain, the 44-year-old resident of Brampton, Ont., carried on and finished her second Boston Marathon in as many years.

"I saw these signs along the [marathon route] — 'pain is temporary, glory is forever.' I thought, forget all this pain and keep going."

Blisters, like the one that plagued Carvalho, are caused by friction. But you don't have to run a marathon to develop them.

Blisters form when fluid from blood vessels leaks into the skin. The fluid — which is usually clear — collects beneath the outer layer of skin forming the typical raised bump.

Burns — including too much exposure to the sun — can also cause blisters anywhere on the body.

A "blood blister" can form when you damage your blood vessels — for example, when you pinch your skin with a pair of pliers.

As Canadian feet shed their winter wear and begin donning flip-flops, sandals and ill-fitting, strappy shoes, more of us will be dealing with the discomfort of friction blisters.

'[Blisters] … are your body's response to injury.'—Dr. Brett Taylor

It was only after the race that Carvalho realized what price her foot had paid. A few days of discomfort. But it could have been much worse.

"[Blisters] caused by friction, like the ones you get when you go on a long hike, are your body's response to injury," Halifax pediatrician Dr. Brett Taylor told CBCNews.ca.

"If you keep hiking on a blister without covering it up, you will end up with an open wound, and ultimately a local infection, which can in extreme cases become hazardous. So simple common sense prevails: if you have a blister due to friction, rest it, cover it, and you won't have to worry about it."

Taylor recommends leaving the blister alone — unless it is extremely painful or making it difficult for you to walk.

You could ask your doctor to lance it, or do it yourself if you carefully follow these steps:

• Wash your hands and the blister with soap and warm water.
• Swab the blister with iodine or rubbing alcohol.
• Sterilize a clean, sharp needle by wiping it with rubbing alcohol.
• Use the needle to puncture the blister. Aim for several spots near the blister's edge. Let the fluid drain, being careful to leave the overlying skin in place.
• Apply an antibiotic ointment to the blister and cover with a bandage or gauze pad.
• After several days, cut away the dead skin, using tweezers and scissors sterilized with rubbing alcohol.

If you see signs of infection around a blister including pus, redness, increasing pain or warm skin, you should call your doctor immediately.

While most blisters are harmless, some are signs that something more serious is happening. Blisters caused by infection are known as "vesicles" or "bullae," and there are several causes, Taylor said.

"Children can get a bacterial infection of the skin [impetigo] that causes small blisters to form, often around the mouth. This can be quite contagious, and spread to other sites on the body and to other children by the child's fingers. The vesicles of impetigo often form in little clusters and can weep fluid. Impetigo is treated with antibiotics."

Other conditions that can lead to blisters include:

• Cellulitis. In this illness, the blisters are often on the edge of a reddened, tender, swollen piece of skin. The child might have a fever, but this isn't necessary. Again, antibiotics are needed to minimize the risk of spread of infection and scarring.
• A viral infection, like chicken pox. This is far less common with mass immunization. Anyone who has a mass of small blisters should be seen by a doctor, but often the treatment is just wait and see.
• An adverse reaction to a drug, or the result of contact with an irritant. In this case, the patient has usually started a new medication, applied a new cream or come in contact with a new chemical, and within a day or two develops a fairly dramatic rash. Generally, stopping the exposure cures the problem.
• Inherited illness that causes the skin to be prone to blister formation. One example is the disease called "epidermolysis bullosa." This illness shows up most commonly in infants and toddlers, and can be very difficult to treat.

You can avoid the nuisance of dealing with blisters by making sure they don't happen in the first place.

'I did feel something in the area. I should have seen it coming.'—Helena Carvalho

• Wear shoes that fit properly.
• If you're prone to blisters, protect the spots they are likely to occur with items such as bandages, tape or moleskin.
• Keep your feet as dry as possible. Wet shoes, boots and socks are blister breeding grounds.
• If you're into endurance events, make sure your shoes are properly broken in.

Carvalho believes her improperly broken-in shoes caused her monster blister.

"I wanted to keep the shoes nice and clean for the race, so I don't think I wore them enough. Just for short indoor training runs — I did feel something in the area. I should have seen it coming."