It triggered a wave of events in which the patient, Gaëtan Dugas, a French-Canadian flight attendant, was erroneously blamed for bringing the human immunodeficiency virus, or HIV, to the United States.
Though Dugas’ disturbing saga has been put to rest, the term “patient zero” lives on, and continues to create confusion and curiosity about how disease spreads.
“Zero is a capacious word,” Richard McKay, a historian at the University of Cambridge in England and a co-author of the Nature study, said last month. “It could mean nothing, but it can also mean the absolute beginning.”
Super-spreaders vs. super-shedders
Many scientists and public health officials are loath to identify those patients and avoid the term “patient zero” altogether, said Thomas Friedrich, an associate professor of pathobiological sciences at the University of Wisconsin-Madison School of Veterinary Medicine.
“Identifying one person as the patient zero, on the one hand may give an incorrect impression about how the disease emerges in the first place and, on the other hand, insinuate that somebody should be blamed for this outbreak, when that’s not really appropriate,” Friedrich said. “Nonetheless, it’s important scientifically and for people and public health to understand index cases so that we know how diseases are coming into a community and how to stop their spread.”
“It is not uncommon for infectious agents to percolate in the environment for years or even decades without detection,” Lipkin said, adding that an agent could enter the human population in more than just one person.
And even after an infectious agent crosses into the human population, some people are more capable of spreading it than others, he noted.
“So it may make more sense to talk in terms of super-spreaders than patient zeroes,” Lipkin said. “Super-spreaders may travel or engage in certain types of behaviors … that result in transmission to large numbers of people.”
For instance, even though Dugas wasn’t the patient zero of HIV, he still may have served as a super-spreader, Lipkin said.
Still others might be super-shedders, individuals who shed many more types of the virus into the environment — and not just through person-to-person contact — than others.
Whether they were truly patient zero, super-spreaders or super-shedders, here are the stories of six people who may have played a role in the spread of deadly diseases in the 20th and 21st centuries.
The real Typhoid Mary
She became known as Typhoid Mary, said Dr. Richard Stein, a research scientist at New York University School of Medicine and adjunct assistant professor at City University of New York, who wrote the book “Super-Spreading in Infectious Diseases.”
“I’m still not sure whether she was only a super-spreader or whether she was also a super-shedder or possibly both,” Stein said.
As more households where she worked developed typhoid fever, Mallon was soon identified as something of a patient zero, even though she never developed the symptoms.
“There are these individuals, like so-called Typhoid Mary, who for one reason or another may be infected with a pathogen and not have that many symptoms but can shed that pathogen in a way that makes it infectious to other people,” Friedrich said.
No one really knows whether Mallon was the true patient zero in the typhoid case or simply a super-spreader or super-shedder. After all, naming a patient zero remains tricky.
“I can’t even think of a time when we’ve actually known an index case,” said Dr. Bertram Jacobs, director and professor of virology at Arizona State University’s School of Life Sciences. The closest we can come is probably the SARS epidemic, he said.
The spread of SARS
The other hotel guests who were exposed to the virus probably traveled to other countries after being infected. In less than four months, about 4,000 cases and 550 deaths from SARS could be traced to Liu’s stay in Hong Kong.
How was Liu infected with SARS to begin with? The hospital where he worked treated SARS, and Liu might have come into contact with the virus through a patient.
“For many zoonotic infectious diseases, the first step involves the species jump, and then if the virus is able to be transmitted directly among humans and no longer needs the animal reservoir for this, it has the potential to unleash an epidemic,” Stein said. “I think that, exploring this from a global perspective, that very first patient would be patient zero.”
‘Contagion’ in real life
“There are plenty of potential patient zeroes out there that get infected with stuff,” said University of Wisconsin-Madison’s Friedrich.
“What’s preventing a new pandemic is not so much that animal viruses just can not infect humans. In fact, the more we look, the more we see that people do get infected with animal viruses all the time,” he said.
The key to an outbreak, Friedrich said, is for those viruses to be transmitted from a single person to more people. That’s exactly what H5N1 avian flu, or bird flu, did.
A new virus emerges
“New epidemics that emerge in humans often come from contact with animal diseases,” Jacobs said. “The viruses that really circulate in humans turn out mostly to be reassortants between viruses, usually from birds and other human viruses.”
A reassortant is the virus that emerges when the genetic material from two or more other viruses infecting a single human or animal host mix together. That mixing is often referred to as a reassortment event, which occurs frequently in nature.
“Avian influenza viruses don’t replicate well in humans, human influenza viruses don’t replicate well in birds, but if a bird virus and a human virus gets into a pig, you can have reassortment and get totally new strains out,” Jacobs said.
A boy who survived swine flu
H1N1 influenza emerged in humans to cause a pandemic in 1918, Friedrich said, and then a similar pandemic hit the world in 2009.
The resulting outbreak serves as an example of how new flu viruses might enter the human population, Friedrich said.
“For the most recent H1N1, it definitely seems like it came from pigs, (even though) they’re ultimately bird viruses,” Friedrich said.
“The term that people use in the flu world is that pigs are ‘mixing vessels,’ in which bird viruses and mammal viruses can mix together and create new combinations that are more likely to infect people than just straight-up bird viruses,” Friedrich explained.
“With these index cases, it’s not just getting infected with an animal virus, but rather, that virus probably has to do things to become transmissible in people, and it has to get out of that first person and get into other people to really become an outbreak,” Friedrich said. “Some viruses can do that, and some can’t. Some can do it but only weakly, and they die out, and some do it really well, and then those become the outbreaks that we hear about.”
Emile and Ebola
One virus long known to aggressively pass from human to human, like a piton in a dangerous race, is Ebola.
He died four days after his symptoms emerged. Within a month, his grandmother, mother and 3-year-old sister died from the disease as well.
As in previous Ebola outbreaks, the virus continued to spread through contact with the body and with human bodily fluids, possibly even after the human died, said Stein, the research scientist at New York University.
“Ebola is interesting, because for this infectious disease, studies found that unsafe burial practices, which involve washing and preparing the body of the deceased, apparently contributed to the infection of many people who were participating in this cultural practice,” Stein said, noting that people could have dispersed a high number of viruses before dying, as well.
‘In the wrong place at the wrong time’
However, it’s still important to keep in mind that “it’s very difficult for us to identify who is a cardinal case in an outbreak,” said Lipkin, the epidemiologist at Columbia University. “There are rare examples of when we do see that happen, like in the Ebola outbreak, because we followed it in real time, but when you try to reconstruct after the fact, it’s easy to be misled.”
And being misled isn’t good for science or humanity, said Jacobs, the virologist at Arizona State University.
“As humans, we sort of want to make tight stories about things, and sometimes that involves blaming or saying, ‘Oh, this person started the epidemic,’ ” Jacobs said. “With history, at least the history of infectious diseases, it’s not that tight. It’s not that clean.
It’s very rare that we can say, ‘This person did it; this person started it.’ Even if they did, most of the time, they weren’t doing anything consciously to start an epidemic. They happened to be in the wrong place at the wrong time.”