Ruth’s hair was gone from her first rounds of chemo, replaced by a collection of wigs that ranged from professional to va-va-voom. She was wearing one from the latter category, with thick red hair. “World’s smallest transvestite,” she had concluded when she saw herself in it the first time.

I honestly thought her bald head looked beautiful, even though she never bought into my whole “you look amazing without hair” strategy. Before chemo started I had gotten my laptop and Googled “sexy bald women.” It went well at first: Persis Khambatta was the first hit. She was the former Miss India, and several of my friends had already proposed her as the model of what Ruth would look like, albeit by her more familiar name “the bald chick in the first Star Trek movie.” Natalie Portman was the next hit, as she appeared in some movie I had never heard of – I was still doing O.K. Third was Jane Curtin as a Conehead. I lost the round.

Now, weeks later, as Ruth wore her red wig, the first milliliters of Taxol flowed.

Drip. Drip. Drip.

From the IV bag, into the tubing, and into Ruth’s left arm. She was undergoing treatment for breast cancer at Memorial Sloan-Kettering, where I also work as an attending physician. Ruth’s nurse, Chirly Denton, stood by. She was holding a medication box containing epinephrine and Benadryl, potentially lifesaving medications in case of a dangerous reaction to the drug.

It is rare, but people do sometimes have a severe allergic reaction when they get the first dose of Taxol. I have colleagues at Sloan-Kettering who recall it happening with horror.

I saw the deadly reaction once myself. I was in my medical training at Johns Hopkins. The patient was in his 70s. I had never met him, but I was called because I was the doctor in charge of cardiac arrests and code blues that day. His heart had stopped a few minutes after the drug began to flow.

Back then Ruth and I were newlyweds, and I probably told her that evening about what I had seen. I did not remind her of it as she and I walked into the treatment area before her first Taxol dose, but the moment was playing loudly in my head.

The patient had incurable lung cancer, but that day it was his treatment that killed him. He had no pulse when I got there. I called the family. His granddaughter answered, and then went to get her grandmother. There is no easy way to give this news, whether to a stranger or to someone you’ve known for many years. And there is no way to ease someone into the reality of death. I introduced myself to the woman for the first time, and told her that her husband had died.

Ruth was not immune to this rare catastrophic event. Her risk was no lower because she was the wife of a doctor at Sloan-Kettering, and squeezing her hand didn’t lower her risk either.

It dripped in, about half a second between each drop.

Ruth’s oncologist is brainy, boyish and no taller than I am (and I was always the last pick on every team). He is methodical in his thinking, quantitatively driven, and like nearly all of the other doctors at Sloan-Kettering, he is direct. But like a batter on a long hitting streak who insists on eating the same lunch or putting on the same T-shirt before every game, Ruth’s oncologist had explained to us earlier that he was going to make his one bow to superstition that day. He proceeded to describe every risk of Taxol in minute detail, insisting that when he performed this ritual none of the complications occurred.

He told us that the reaction was incredibly rare, but noted that “it has happened here.”

Ruth reflexively responded that she didn’t feel protected by long odds anyway.

Each time we heard “that rarely happens,” we knew that whatever it was – fevers requiring hospital admission, mouth ulcers that would stop you from eating, overpowering fatigue, hair coming back a different color – it could actually happen. Each time the odds were in our favor, but the odds on the other side were not zero.

The surgery had gone pretty well. The chemo up until that point had the predicted side effects. But each new day, each new test and each new treatment carried new dangers. Clearing one hurdle did not mean the next would be lower.

The whole harrowing experience up to that point had in one way or another just been a dizzying progression of numbers and chances and probabilities culminating at that moment – a colorless fluid easing its way into Ruth’s circulation that had a hair’s breadth of a chance killing her.

The Taxol dripped in. Five minutes, 10 minutes, 15 minutes went by, and Ruth’s nurse stepped out. The danger period had passed, and the spin of the roulette wheel had come to a favorable stop. We had avoided the extraordinarily rare and unthinkable.

Forty-five minutes to go with this dose, followed by several more in the coming weeks. Each less risky than the one before. None without risk.

The odds that Ruth would get breast cancer before she reached age 43 were about 100 to 1. Odds seem a lot different once you’ve come up on the short end.

Dr. Peter B. Bach is an attending physician at Memorial Sloan-Kettering Cancer Center in New York City. His essays, about his wife’s breast cancer, appear Tuesdays on the Well blog.