Although the 20th century gave us plenty of reason to be skeptical of the so-called “herd mentality,” scientific research in recent years has uncovered how the wisdom of crowds can actually benefit the individual.

One area of medicine, in fact, is built on it.

“Herd immunity” is what happens when enough of the population is vaccinated and acquires immunity against certain diseases to ensure that those who aren’t vaccinated will remain healthy. As immunization experts will tell you, it’s the reason some of the world’s worst diseases (such as polio, which persists on the Indian subcontinent and in other poor countries) are virtually nonexistent in the United States.

On a deeper level, however, some might argue that herd immunity demonstrates how our bodies are all connected. How the notion of independence, at least in the physical sense, is a myth. And how personal decisions regarding vaccinations are rarely made in a vacuum.

As long as there have been vaccines, there have been people who question them. But this isn’t a story about the vaccination debate. It’s about a mother who experienced the kind of personal heartbreak that few others ever will, radically altering her perceptions of public health, the science of vaccines, and what it means to make an informed decision.

The woman is Veronica McNally, and her office at the Franny Strong Foundation in West Bloomfield is covered in purple carpet. She’s an attorney by trade, but more recently, she’s become a public-health advocate.

It’s a second job she never planned on having. She never even liked the color purple. But last February, McNally decided to paint her newborn daughter’s bedroom purple instead of pink. At the time, she didn’t know what made her do it. But three months later, as she drove to Francesca’s funeral, she looked up awareness colors for the bacterial respiratory infection pertussis. One of them was purple, McNally says, almost in awe of how the color, like some sort of call to arms, had dominated her daughter’s short life. Francesca was one of 17 infants nationwide who had died of pertussis (commonly known as “whooping cough”) by the end of 2012, and her death would end up pushing the McNally family to the center of an outbreak — and a cause.

“I’ve seen infants who are 6 weeks old die of pertussis in the hospital, and it is so heartbreaking because it is so preventable,” says Dr. Matthew Davis, describing the “overwhelming” swelling of a baby’s lungs and airways during the last stages of pertussis. An associate professor of pediatrics, internal medicine, and public policy at the University of Michigan, Davis says that “even our best medicines aren’t equipped to deal” with that kind of swelling.

McNally and her husband, Sean, had never heard about the dangers of pertussis in babies, or that more people in the United States — usually infants less than three months old — die from it than any other vaccine-preventable disease.

They had no idea that parents can easily pass it to their babies, and that roughly 90 percent of American adults are no longer immune because they haven’t gotten an adult booster shot. They also had no idea of the outbreak that was spreading, leading the Centers for Disease Control and Prevention to declare last July that the U.S. was on track to experience the worst pertussis epidemic in 50 years.

Now, as McNally sits in the office of the foundation that she and Sean created in their daughter’s memory, she speaks with laser-like precision on the history, statistics, and prevention of pertussis. For all intents and purposes, she is an expert. She’s also determined to keep what happened to her family from happening to anyone else’s.

“It’s what I think of every second of every day,” she says.

But how, exactly, did this happen? McNally is calculated — almost clinical — in describing the details of multiple doctor visits, unrecognized symptoms, and a stark confrontation with how vaccines work. She is quick not to place blame. It wouldn’t be a stretch to wonder if she has found peace. “But is it hard to deal with? Absolutely,” McNally says. “I mean, I had a baby girl (last February), and I don’t have her anymore.” She also has two young sons who lost their only sister. “We’ve not shown them sheer and utter devastation,” she says.

It started with a runny nose, and then a cough. One so intense, McNally says, that she can still remember feeling it in her lungs as she drove in her car late last March. “I remember thinking, ‘My God, this is a bad cough.’”

Several weeks before, she couldn’t have felt more alive. She had just given birth to her first daughter, Francesca Marie. “I always felt like I was holding an angel,” McNally says, pointedly insisting how she had equally admired her two sons who came before. “I was over the moon for them. But when this little girl was born, it was euphoric.”

By the time McNally went to the doctor on April 18 to address her cough, the doctor told her not to worry. All three of her children had suffered from respiratory syncytial virus (RSV), a common, but generally mild respiratory infection, in mid March. They had all gotten better, and McNally simply figured she had caught the tail end of it.

Then, around May 3, as McNally continued to deal with her own coughing issues, the coughing started again in one of her sons. A pediatrician diagnosed him with croup, another common respiratory infection in children. “But I was skeptical about that diagnosis,” McNally says, noting that the doctor did not perform a test for confirmation. Francesca was also starting to cough, and McNally began to seriously worry that her infant daughter might catch whatever it was that was plaguing the family.

“We’re kidding ourselves if we think we’re going to be protected … through herd immunity alone.” – Dr. Matthew Davis

“I waited for the sky to fall,” she says.

On May 10, during a vaccination appointment for her two sons, a different doctor agreed to see Francesca. She listened to her breathing and confirmed that she had “definitely caught something.” She also prescribed antibiotics for McNally’s son who had been diagnosed with croup, but wanted to wait to see if Francesca’s conditioned worsened before deciding on a treatment. But by May 12, Francesca had worsened to the point that McNally decided to take her to the emergency room.

In addition to her cough, Francesca was lethargic and didn’t want to eat. Tests for RSV and the flu came back negative, however, so the ER doctor didn’t seem worried. “He told us that kids are very simple, and you know when they’re really sick,” McNally recalls. “He looked at her sleeping on the bed, and he said she’s OK.”

McNally went home, having been told by several doctors at that point that whatever it was, Francesca would be fine. “I spent Mother’s Day just trying to get my infant daughter very comfortable — and she was miserable,” McNally says. “I kept thinking to myself, ‘How is she ever going to get over this?’”

It wasn’t until May 14 that a doctor finally speculated that Francesca had pertussis. An extremely high white-blood-cell count seemed to confirm the diagnosis, as well as the severity of the infection. McNally wanted to take Francesca to the ER right away, but the doctor advised against it, saying it would only expose her daughter to other diseases.

Ignoring the doctor’s warning, McNally and her husband took Francesca back to the ER for a second time, where doctors immediately admitted her to the pediatric intensive care unit. The parents had a terrible feeling about what was happening, and time began to move at warp speed.

“That was May 14,” McNally says. “Francesca was intubated on May 15, and she died on May 17.”

When McNally later learned just how contagious and deadly pertussis can be, she was shocked. She and Sean would also learn too late about Tdap, the pertussis vaccine for teens and adults that could’ve shielded Francesca from the disease. McNally had never gotten the adult vaccine. Sean got it during a routine medical exam a few years ago, but only because of its tetanus component. This would end up making it safe for him to be around his infant daughter. For McNally, she’s had to come to terms with the possibility that her own body, to which she had held her baby so close, might have been the catalyst for the tragic chain of events.

“Although it was never confirmed,” she says, “what I believe now is that I had pertussis, and that I gave it to my child.”

Tdap is, in fact, the only way to protect children as young as Francesca from getting the disease, a message that has gained momentum only with the latest outbreak. This is because infants can only begin to develop their own immunity after about six months of age, when they start to receive the first in the series of pertussis-related vaccines. Even then, children aren’t fully immune until they’ve received the full series of shots — something that can go unchecked if a child is on a delayed vaccination schedule. Tdap for adults, then, would become a critical tool for protecting everyone when it was developed in 2006. According to the latest data collected by the CDC, however, just 8 percent of the adult population in the U.S. had received the Tdap vaccine by 2010.

“We’re still way behind where we need to be,” Davis says.

The adult vaccination rate for pertussis may be low, but it’s not necessarily surprising. Today, researchers are continually trying to better understand the nature of whooping cough — as they’re doing with all vaccine-preventable diseases — and, as a result, vaccination strategies are continuing to evolve.

McNally and others agree that this is why many doctors, let alone the majority of the general population, may not be aware of the newer pertussis protocols. Those include one of the most dramatic recommendations from the CDC announced last December: that during each pregnancy, women should receive the Tdap vaccine — regardless of prior history — so that pertussis antibodies can be passed to growing fetuses, furthering a baby’s chances of fighting off infection.

“What I believe now is that I had pertussis, and that I gave it to my child.” – Veronica McNally

Associated with violent fits of coughing that sometimes produce a “whooping” sound, pertussis affects more infants than all other age groups. But because coughing can be attributed to so many other conditions, it often goes undiagnosed and underreported. Still, there were more than 40,000 confirmed cases across the U.S. by the end of 2012, representing a 300 to 700 percent increase depending on the area over prior years. Highly contagious, pertussis spreads through the microscopic droplets emitted when an affected person coughs. When a healthy person inhales the bacteria, it infects the cells in the lungs and the airways. The toxin produced soon begins to affect the normal function of those cells, causing a heavy production of mucus. Regardless of antibiotic treatment, the patient must produce new airway cells unaffected by the toxin to fully recover, which can often take six weeks or more. Pertussis is so dangerous in babies because the heavy mucus and lingering cough can clog their airways. While this is rarely deadly for adults whose airways are large enough to compensate, a baby’s relatively small airways, in most cases, are simply too small.

“There are effective antibiotics that help adults get over pertussis,” Davis says. “But we lack a surefire therapy for pertussis in babies.” And once the swelling starts in their airways, it’s hard to make it stop.

Despite its deadly potential, pertussis persists as an “old-fashioned” disease — the kind of thing that might have killed people regularly a hundred years ago. In today’s world of advanced medicine, it’s widely believed that whooping cough is no longer even a threat to public health. But the pertussis vaccine has been part of the CDC’s recommended routine vaccination schedule for decades.

It’s a “common misperception, ” Davis says, that people just don’t get whooping cough anymore. In reality, he says, pertussis outbreaks have been around for as long as the disease and the vaccine, and researchers still don’t know why these incidents tend to rise and fall in cycles.

But there are some things they do know.

Communities with lower vaccination rates tend to experience worse outbreaks, which, experts believe, has led to higher incidences of whooping cough in places like California and Washington. These are two states, it turns out, where a higher percentage of the local population deviates from recommended vaccination schedules or declines vaccinations altogether (on a national level, various reports estimate that approximately 15 percent of the population today chooses alternative or delayed vaccination schedules instead of following the CDC’s standard recommended schedule).

It’s also been known for some time that the herd-immunity threshold for whooping cough is higher than it is for other vaccine-preventable diseases. For example, only about 85 percent of the population needs to be vaccinated for polio or smallpox to protect those who aren’t vaccinated. By contrast, in order for herd immunity to take effect, about 94 percent of the population must be vaccinated for pertussis.

“Herd immunity is a wonderful thing,” Davis says, “but in some ways it’s been so successful that people have started to misinterpret it to mean that we don’t need to be as vigilant anymore about vaccination. In fact, we know that even a 5 to 10 percent decline in vaccination levels in this country could bring back other diseases that we haven’t seen in decades.

“We’re kidding ourselves if we think we’re going to be protected from pertussis through herd immunity alone.”

More recently, researchers have learned that pertussis immunity wanes in adulthood, even if someone received all the recommended vaccines as a child. This is what led, largely, to the development of Tdap. According to the CDC, adults need to get a booster every 10 years. This is even more critical if an adult expects to be around an infant for any length of time.

It is this complex and deadly nature of pertussis that McNally says compelled her to see that Francesca’s death was not in vain. Promoting prevention of pertussis through better awareness — and especially through the Tdap vaccine in adults — is the Franny Strong Foundation’s main mission.

“No one needs to die of pertussis,” McNally says.

On a more personal level, McNally says her family’s tragedy has led her to consider the whole picture in the vaccine debate. One of McNally’s sons was on a delayed vaccination schedule because of her own concerns about his body’s tolerance. Her pediatrician, she says, was tolerant of her views, failing to provide standard, widely accepted medical evidence for the safety of vaccines.

In the wake of Francesca’s death — and with the overwhelming reminder that knowledge of a simple vaccination protocol could have saved her daughter — McNally just wants to make sure that people are worrying about the right things.

“What I’m saying is, if any parent is going to choose to delay vaccines, or question them, then much more significant conversations need to be going on,” she says. “I was a cautious and naive parent.”