A pandemic is an infectious disease in human populations that has spread worldwide and affected many people in almost all countries, said Thomas Campbell, professor of medicine in the Division of Infectious Diseases at the University of Colorado School of Medicine.
“What we’re talking about is the global spread of an infectious disease,” he said. (This is why Ebola doesn’t meet the definition; the cases are concentrated in just a few countries, with a few others having a handful of cases.)
The spread doesn’t have to be fast for a disease to be considered a pandemic — HIV is a recent example of a slow-spreading pandemic — and the illness doesn’t have to be deadly. “Severity is not part of the pandemic definition, but we are more concerned about the ones that are severe and have the most potential to cause death or harm,” Campbell said.
What makes a particular pathogen dangerous? One factor is a prolonged incubation period or an asymptomatic phase. For example, most people don’t know when they are first infected with HIV, so they can easily spread the illness before they experience any symptoms of it themselves.
Are pandemics increasing? It’s not clear.
“We’re at the most risk we’ve ever been for pandemics. We’re doing so many things on the planet that cause them to emerge, and we’re so connected to everywhere else,” said Peter Daszak, president of the EcoHealth Alliance, a nonprofit focused on global health issues.
On the other hand, international travel has been happening for centuries, albeit more slowly than today, Campbell said. The introduction of smallpox by the Europeans into the Native American population is an example of a disease that spread that way.
“The difference between pandemics today and pandemics from centuries past is that now we have a much better scientific understanding of the factors that cause pandemics,” Campbell said. “We’re much more able to maybe not prevent the pandemic but to lessen the effects.”
For example, he said, although the bubonic plague still exists and sickens some individuals, better living and sanitary conditions keep it from wiping out millions of people as it did in the Middle Ages. In addition, a strong public health infrastructure allows outbreaks to be identified and stopped early, and victims can be treated with antibiotics. “It’s a combination of knowing the science and having good public health and medical care,” Campbell said.
It’s not possible to know exactly what types of pandemics are in our future or when they will occur. However, it’s possible to evaluate the most likely ways for them to originate and potential candidates.
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There are two main kinds of pandemic, Campbell said. Some originate from an animal reservoir — there are infections that occur in animals that cross into people and then spread through the human population. This happens frequently with influenza, for example.
These diseases don’t usually kill the animals that are their hosts. “From the pathogen’s standpoint, it’s not a good strategy to kill your host as fast as you can,” said
Robert T. Schooley, professor of medicine and head of the Division of Infectious Diseases at the University of California, San Diego.
But they can be deadly when they spread to humans.
“What is worrisome is that when there is that cross-species transmission, if humans haven’t been exposed to it, there may not be any herd immunity,” said Campbell. Herd immunity, in which some members of a population are immune to an illness, helps to slow its spread. The Spanish flu epidemic of 1918 is an example of a flu strain that rapidly spread around the world once it appeared in humans.
How do these viruses make the jump from animals to people? One common way is through the food supply or in the process of butchering animals for food.
“Pandemics generally originate in the tropical countries where there’s a lot of wildlife diversity and human activity,” Daszak said. For example, SARS (short for severe acute respiratory syndrome) came from bats and has probably existed in bats for a long time since it doesn’t kill them. Eventually people got infected and it started to spread.
“Once a pathogen is able to go from human to human and it gets into a city, people get on planes and travel,” Daszak said.
“There are lots of different animals out there, and they have lots of different viruses and bacteria,” said Campbell. “And we don’t know about all of them.”
The second type of pandemic comes from a disease that exists only in human hosts. These are “less troublesome,” Campbell said, because they’re better known. “We’re able to know about them and take measures like developing a vaccine to keep them from spreading.”
The Candidates
What will the next pandemic be?
“If we knew what it would be, we’d be preparing for it,” Schooley said.
Still, it’s possible to consider some possibilities, with some more likely to pose a threat than others.
Measles and pertussis
Experts agree that although resistance to vaccinations has caused some outbreaks of these illnesses, they are not likely to cause a pandemic since we have vaccines and understand how to stop the diseases.
“It would be very unlikely that that would become a pandemic as long as we maintain adequate vaccination rates,” Campbell said.
Lower vaccination rates do carry some risk, though, both to the people who are not vaccinated and to the population as a whole, since a smaller fraction of the society will be immune, Schooley said.
Ebola
Ebola is not a pandemic today — “the vast majority of countries around the world have not had a single case,” said Campbell — although it’s definitely an epidemic in West Africa.
And it’s not clear if Ebola is a strong candidate to cause a pandemic. “Ebola is not very smart: It causes an acute disease and kills a large fraction of its infected people,” Schooley said. There is a very short period, perhaps none at all, when an infected person can transmit the disease but does not yet have symptoms.
“And once you get over it, it’s gone,” Schooley said. “It doesn’t have a latent form that allows it to be in a human over long periods of time and get transmitted to other people who don’t realize the person is a carrier.” This makes it easier to halt the spread, especially when the disease is recognized early and the health-care system can respond.
Monkeypox
Monkeypox, a relative of smallpox, came to the United States a few years ago from Africa with some imported pets. “The outbreak got controlled, and they banned the importation of rodents from West Africa,” Daszak said. “But the big concern was, what if it had gotten into wild populations of groundhogs? That’s really hard to get rid of.”
Rift Valley fever
This fever, carried by animals such as antelope and sheep, can kill people. It can also be transmitted by mosquitoes. “If that gets into the United States, it’s a virus that could persist in our animals and then be very difficult to eradicate,” Daszak said.
Chikungunya
This virus is working its way through the Caribbean, and Schooley suspects it will be seen more often in the Southeast and eventually the Southwest of the United States. Like West Nile virus, it is carried by mosquitoes.
Dengue or malaria
Climate change may affect where pandemics can take hold. “Mosquitoes that used to be restricted to warmer climates are finding places where they can propagate more effectively,” said Schooley.
This could lead to a resurgence of illnesses like dengue and malaria in places where they haven’t been common.
Influenza
Flu pandemics happen when a new strain of flu — that people are not vaccinated against or immune to — enters the population. They have happened in the past, and “we can certainly expect” influenza pandemics in the coming years, Campbell said. In fact, this is one type of pandemic that has received a lot of preparation. (See the sidebar on page 18 for resources.)
Unknown virus
It’s quite possible, of course, that the next pandemic will be an illness no one has yet identified. The biggest threat, Daszak said, is a disease that is spread by mosquitoes or that can be transmitted through the air.
What to Do
What should those in charge of preparing for pandemics — including public health officials and emergency managers — be doing?
Emergency managers are on the front lines for these sorts of events, Daszak said. “Pandemics are like any other threat or danger: It’s all about managing risk and getting ready to deal with it when it happens.”
One key is that management is a lot easier and more cost-effective at the beginning or even before the outbreak.
Looking at trends over the past 60 years to see where diseases such as West Nile virus, SARS and Ebola have emerged and how they have spread, some patterns become clear. “The primary drivers of pandemics are things that we do on the planet: chopping down forests, more intensive farming and increasing travel networks,” Daszak said.
On a broader level, experts warn that actions people take now could increase the risks of, and from, pandemics.
For example, people build logging camps in tropical forests and send in workers, telling them they can hunt for their food. “We’re effectively placing people right in the middle of a virus hot spot and telling them to go out and hunt wildlife,” said Daszak. That’s a classic way that previous pandemics have emerged.
The creation of more intensive farms, particularly for poultry, is another cause for concern, Daszak said. “To a virus, a very intensive chicken farm is just a big pile of protein waiting to be infected.”
Of course, it’s not necessarily practical to stop practices, from intensive farming to international travel, that could help pandemics emerge. “The truth is, this is what we do: We need food, we need to travel and have a trade network,” Daszak said. “We should get ready for the risk and not just hope it doesn’t happen.”
For example, when companies build roads into a forest, they should also build a food supply chain to reduce the risk that workers will go hunting on their own.
And a good surveillance system in the health-care system is crucial.
“We’re much better prepared now to deal with pandemics than we have been in the past,” Schooley said. Sophisticated microbiological techniques can identify organisms we didn’t previously know about.
Improving the health-care systems, especially in parts of Africa, Asia and Central America where new pathogens often originate, is another key to preparedness. The Ebola outbreak does show how important a good health-care system is in preventing the spread of a disease. When the current outbreak started, authorities initially believed it was cholera. The Ebola outbreak might not have reached the U.S. if the health-care system in West Africa had been more robust. Even in the U.S., when an early victim appeared in an emergency room, he was initially sent home.
Public and private organizations should have pandemic plans, said Bo Mitchell, former police commissioner of Wilton, Conn., and founder of 911 Consulting. The plan should be “evergreen,” he said. “It doesn’t matter what the virus is called.”
Some of these plans may cover questions that were raised with the Ebola cases: How do you protect caregivers? Can health-care workers be forced to provide this care? Should they be quarantined?
Another key issue with pandemic planning is economics. “The economy has to keep on going,” Mitchell said. Most cities do not have enough food to feed the population if a quarantine won’t allow deliveries or employees to go to work for an extended period.
“We don’t know what the next pandemic will be or where it will come from,” Campbell said, “but we have to be prepared.”
