The Boston Marathon, held every year on Patriots' Day — the third Monday in April — is one of the world's oldest marathons and some might say one of the the world's most revered foot races. It was on this stage in 1967 that a young college student named Kathrine Switzer made history by becoming the first woman to officially enter and complete the race. Today, on the 50th anniversary of her momentous run, 70-year-old Switzer will again toe the line of the Boston Marathon, honoring female runners everywhere.

Take a look around the field at a marathon or 5K today and you'll see a diverse group of people. Men and women of all ages and abilities come together at these events to put one foot in front of the other and celebrate their love of running. But it wasn't always that way. Running at one time was reserved for the elites — and that meant only the fastest, and only the men.

It's hard to believe, but 50 years ago, even doctors warned against the potential health dangers for women who wanted to run. Women were told they were too delicate and frail to endure even a simple jog around the block, let alone a 26.2-mile marathon. Following these "health guidelines," women were not allowed to enter marathons, and certainly not one as prestigious as the Boston Marathon.

But if there was one thing that 20-year-old Kathrine Switzer knew in 1967, it was that running made her feel good. It gave her confidence, and made her feel strong and fearless. After all, she had been running at least one mile every day since she was 12 years old. When she went off to college, at 18, she started running three miles every day. When she asked the track coach at Syracuse University if she could join the team, he told her it was against the rules for women to run on the track team, but that if she wanted to practice with the team, she could.

It was during those practices that Switzer met assistant track coach Arnie Briggs, who regaled her with stories about the celebrated Boston Marathon. When Switzer decided to run it, Briggs repeated the line that it would be bad for her health because she was a woman. Not to be deterred, Switzer, who was already running 10 miles a day at this point, convinced Briggs to help her train for the race under the condition that she complete a 26.2-mile training run before he allowed her to enter. She ran 31 miles instead.

Making a bib number famous

So Switzer sent in her registration form, and her $2 entry fee, and signed up for the Boston Marathon using the pseudonym she always used in her writing as a journalism student, K.V. Switzer. It's worth noting that in the two previous years, a woman named Roberta "Bobbi" Gibb had attempted to enter the Boston Marathon but was rejected by race officials because she was a woman. (Gibb ran the race anyhow, even without a bib number.) But as Switzer's name didn't raise any alarm flags with race officials, she was granted a bib (#261) and on that cold April day in 1967, she pinned it on and began running the Boston Marathon.

Switzer's run didn't initially attract any attention. She was bundled up and just running along, but around halfway through the race, a press truck come up alongside the runners and the media took notice of the woman on the course. So did race director Jock Semple, who had been riding along on the press truck. Semple was furious that a woman would try to make a mockery of his race, and in his rage he jumped off the press truck and attempted to tackle Switzer and rip the bib number off her.

That moment was captured by the press in the iconic photo series shown above. That, and the fact that Switzer went on to complete the race without passing out, damaging her lady parts, or becoming a lesbian - all "dangers" associated with running as a woman in 1967, cemented Switzer's spot in marathoning history, and paved the way for women everywhere to take part in the sport.

Today, 50 years later and at the age of 70, Switzer will once again run the Boston Marathon. But this time, instead of being attacked by the race director, she will be honored for her contribution to the sport. And she also won't be alone. Rather, she will run alongside thousands of women from around the world who have her to thank for their chance to secure a bib.