Two reasons for that, he explained. It's not just the high mutation rates but also the fact that their population sizes are huge. "Those two things put together mean you'll produce more adaptive change," he said.
RNA viruses replicate quickly, generating big populations of viral particles within each host. Stated another way, they tend to produce acute infections, severe for a short time and then gone. Either they soon disappear or they kill you. Eddie called it "this kind of boom-bust thing." Acute infection also means lots of viral shedding—by way of sneezing or coughing or vomiting or bleeding or diarrhea—which facilitates transmission to other victims. Such viruses try to outrace the immune system of each host, taking what they need and moving onward quickly, before a body's defenses can defeat them. (The HIVs are an exception, using a slower strategy.) Their fast replication and high rates of mutation supply them with lots of genetic variation. Once an RNA virus has landed in another host—sometimes even another species of host—that abundant variation serves it well, giving it many chances to adapt to the new circumstances, whatever those circumstances might be.
Most DNA viruses embody the opposite extremes. Their mutation rates are low and their population sizes can be small. Their strategies of self-perpetuation "tend to go for this persistence route," Eddie said. Persistence and stealth. They lurk; they wait. They hide from the immune system rather than try to outrun it. They go dormant and linger within certain cells, replicating little or not at all, sometimes for many years. I knew he was talking about things like varicella zoster, a classic DNA virus that begins its infection of humans as chickenpox and can recrudesce, decades later, as shingles. The downside for DNA viruses, he said, is that they can't adapt so readily to a new species of host. They're just too stable. Hidebound. Faithful to what has worked in the past.
The stability of DNA viruses derives from the structure of the genetic molecule and how it replicates: It uses the enzyme DNA polymerase to assemble and proofread each new strand. The enzyme employed by RNA viruses, on the other hand, is "error-prone," according to Eddie. "It's just a really crappy polymerase," which doesn't proofread, backtrack, or correct erroneous placement of those RNA nucleotide bases, A, C, G, and U. Why not? Because the genomes of RNA viruses are tiny, ranging from about 3,000 nucleotides to about 30,000, which is much less than what most DNA viruses carry. "It takes more nucleotides," Eddie said—a larger genome, more information—"to make a new enzyme that works." One that works as neatly as DNA polymerase does, he meant.
And why are RNA genomes so small? Because their self-replication is so fraught with inaccuracies that, given more information to replicate, they would accumulate more errors and cease to function at all. It's sort of a chicken-and-egg problem. RNA viruses are limited to small genomes because their mutation rates are so high, and their mutation rates are so high because they're limited to small genomes. In fact, there's a fancy name for that bind: Eigen's paradox. Manfred Eigen is a German chemist, a Nobel laureate, who has studied the evolution of large, self-replicating molecules. His paradox describes a size limit for such molecules, beyond which their mutation rate gives them too many errors and they cease to replicate. They die out. RNA viruses, thus constrained, compensate for their error-prone replication by producing huge populations and achieving transmission early and often. They can't break through Eigen's paradox, it seems, but they can scoot around it, making a virtue of their instability. Their copying errors deliver lots of variation, and variation allows them to evolve fast.
"DNA viruses can make much bigger genomes," Eddie said. Unlike the RNAs, they're not limited by Eigen's paradox. They can even capture and incorporate genes from the host, which helps them to confuse a host's immune response. They can reside in a body for longer stretches of time, content to get themselves passed along by slower modes of transmission, such as sexual and mother-to-child. "RNA viruses can't do that." They face a different set of limits and options. Their mutation rates can't be lowered. Their genomes can't be enlarged. "They're kind of stuck."
What do you do if you're a virus that's stuck, with no long-term security, no time to waste, nothing to lose, and a high capacity for adapting to new circumstances? By now we had worked our way around to the point that interested me most. "They jump species a lot," Eddie said.
Whence do they jump? From one species of primate to another, from one rodent to another, from a prey animal into a predator, and so on. Such leaps probably occur often in the quiet isolation of forests and other wild habitats, and usually they go undetected by science. But sometimes the leap is from a nonhuman critter into a human. Then we notice.
The kind of animal that harbors a given virus is known as its reservoir host. Could be a monkey, a bat, maybe a rat. Within its reservoir host the virus lives quietly, in a sort of long-term truce, causing no obvious symptoms. Passage from one kind of host to another is called spillover. In the new host, the old truce doesn't apply. The virus may turn aggressive and virulent. If the new host is human, you've got a newly emerged zoonotic disease.
Spillover to humans, as Eddie Holmes noted, occurs more often among RNA viruses than other bugs. It brings creatures such as Lassa (first recorded in 1969), Ebola (1976), HIV-1 (inferred in 1981, isolated in 1983), HIV-2 (1986), Sin Nombre (the infamous American hantavirus, 1993), Hendra (1994), avian flu (1997), Nipah (1998), West Nile (1999), SARS (2003), and swine flu (2009) into people's lives. Marburg is just another of the leaping threats, rare but dramatic in its impact on humans. Why are these spillovers happening, ever more frequently, in what seems a drumbeat of bad news?
To put the matter in its starkest form: Human-caused ecological pressures and disruptions are bringing animal pathogens ever more into contact with human populations, while human technology and behavior are spreading those pathogens ever more widely and quickly. In other words, outbreaks of new zoonotic diseases, as well as the recurrence and spread of old ones, reflect things that we're doing, rather than just being things that are happening to us.
We have increased our human population to the level of seven billion and beyond. We are well on our way toward nine billion before our growth trend is likely to flatten. We live at high densities in many cities. We have penetrated, and we continue to penetrate, the last great forests and other wild ecosystems of the planet, disrupting the physical structures and the ecological communities of such places. We cut our way through the Congo. We cut our way through the Amazon. We cut our way through Borneo. We cut our way through Madagascar. We cut our way through New Guinea and northeastern Australia. We shake the trees, figuratively and literally, and things fall out. We kill and butcher and eat many of the wild animals found there. We settle in those places, creating villages, work camps, towns, extractive industries, new cities. We bring in our domesticated animals, replacing the wild herbivores with livestock. We multiply our livestock as we've multiplied ourselves, establishing huge factory-scale operations that contain thousands of cattle, pigs, chickens, ducks, sheep, and goats. We export and import livestock, fed and fattened with prophylactic doses of antibiotics and other drugs, across great distances and at high speeds. We export and import wild animals as exotic pets. We export and import animal skins, contraband bushmeat, and plants, some of which carry hidden microbial passengers. We travel, moving between cities and continents even more quickly than our transported livestock. We visit monkey temples in Asia, live markets in India, picturesque villages in South America, dusty archaeological sites in New Mexico, dairy towns in the Netherlands, bat caves in East Africa, racetracks in Australia—breathing the air, feeding the animals, touching things, shaking hands with the locals—and then we jump on our planes and fly home. We provide an irresistible opportunity for enterprising microbes by the ubiquity and sheer volume and mass of our human bodies.
Everything just mentioned falls under this rubric: the ecology and evolutionary biology of zoonotic diseases. Ecological circumstance provides opportunity for spillover. Evolution seizes opportunity, explores possibilities, and helps convert spillovers to pandemics. But "ecology" and "evolutionary biology" sound like science, not medicine or public health. If zoonoses from wildlife represent such a significant threat to global security, then what's to be done? Learn more. RNA viruses are everywhere, as Eddie Holmes has warned, and science has identified only a fraction of them. Fewer still have been traced to their reservoir hosts, isolated from the wild, grown in the lab, and systematically studied. Until those steps have been achieved, the viruses in question can't be battled with vaccines and treatments. This is where the field and laboratory scientists—veterinary ecologists, epidemiologists, molecular phylogeneticists, lab virologists—come in. If we're going to understand how zoonoses operate, we need to find these bugs in the world, grow them in cell cultures the old-fashioned way, look at them in the flesh, sequence their genomes, and place them within their family trees. It's happening, in laboratories and at field sites all over the world; but it's no simple task.
Astrid Joosten wasn't the only person in recent years to die of Marburg. In 2007, a year before her visit to Uganda, a small outbreak occurred among miners in roughly the same area. Just four men were affected, of whom one died. All of them worked at a site called Kitaka Cave, in the southwestern corner of Uganda.
Soon after the news of the affliction got out, in August 2007, an international response team converged on Uganda to assist and collaborate with the Ugandan Ministry of Health. The group included scientists from the Centers for Disease Control and Prevention (CDC) in Atlanta, the National Institute for Communicable Diseases (NICD) in South Africa, and the World Health Organization (WHO) in Geneva. From the CDC there was Pierre Rollin, an expert on the filoviruses and their clinical impacts. Along with him from Atlanta had come Jonathan Towner, Brian Amman, and Serena Carroll. Pierre Formenty had arrived from WHO; Bob Swanepoel and Alan Kemp of the NICD had flown up from Johannesburg. All of them possessed extensive experience with Ebola and Marburg, gained variously through outbreak responses, lab research, and field studies.
Let’s see how long have bacteria’s and viruses been on Earth and how little time have humans been on Earth by comparison, multiplied by the impossible volume to calculate in variety of bacteria’s and viruses that already exist on Earth and are ever changing, oh humans could have another plague tomorrow, anytime really.
Every day is a blessing!
The question is not when, not if, and not even how such a pandemic takes place.
The real question is, "is it a bad thing that it is coming?"
Human population growth will continue until a resource depleation is not met by explotive innovation. We will run out of food, air, water, space, etc - and then population colapse. Many times, such a milestone will be overcome (modern fertilzer and the green revolution inceasing food, or oil shale increasing energy).
Thus, if humans find ways to exploit resources beyond our planet, then beyond our start, then beyond our galaxy - we could continue as a species exponentially throughout reality.
If we do not, however, keep innovation at pace with consumption (a result of population), then there will be an inevitable population crash.
As our population increases, not in number but in density, we become more succeptable to transmission of disease. HIV wasn't a risk when it transmitted into a few villages and died out after a generation or two - it was when it moved into the larger planetary population that problems occur.
If such a disease, however, strikes and reduces our population at regular intervals, without decimating society, then it only allows humanity to progress longer in development on the resources at hand.
If such a disease plagues mankind, and sets him back several decades, so long as the current level of progress is preserved for future renaissance, humanity benefits in the long run (the plague had many positive outcomes for Europe in the long view).
The only real risk is a disease so destructive that it sets mankind's development back in permanent ways that take more generations to discover again than they did to begin with.
Speaking of killing off primates, here is an interesting side link of "...The world's 25 most endangered primates have been revealed in a new report released today (Oct. 15, 2012) at the UN's Convention on Biological Diversity COP11...”
As you read this article, you will find, it is the destruction of their environment that is most lethal to the primates.
Oh, by the way, we humans are a primate species too...
"To put the matter in its starkest form: Human-caused ecological pressures and disruptions are bringing animal pathogens ever more into contact with human populations, while human technology and behavior are spreading those pathogens ever more widely and quickly. "
i think that is the most insightful part of the article.
What the scientists are missing is that bacteria have a consciousness. The idea that without a brain, you aren't intelligent is wrong, and equating brain size to cognition is also a false premise.
Many "medicine men" worldwide talk of disease "spirits". Human consciousness has barely begun to scratch the surface of our potential, and virtually all of our best medicines have come from nature. Discounting their ideas as ignorant or superstitious is idiocy.
Areas where the environment is most destroyed, like certain rivers in europe , and with high populations are likely driving the naturally occurring bacteria out of their natural habitat and into human populations. I believe most of the superbugs are around there as well, accelerating their evolution with antibiotics.
My best guess is that the rise of pandemic and nasty organisms is directly linked to how much the local populations of humans are out of balance with their environment. Correct the environment, correct the risk of pandemics. The sewers of london serve as a surrogate ecosystem, housing rats and god knows what else. Many rivers that existed before london was built dont exist anymore.
Summed up best in one of my favorite quotes, "what you do to the earth, you do to yourself"
"I'd like to share a revelation that I’ve had during my time here. It came to me when I tried to classify your species, and I realised that humans are not actually mammals. Every mammal on this planet instinctively develops a natural equilibrium with the surrounding environment; but you humans do not. Instead you multiply, and multiply, until every resource is consumed. The only way for you to survive is to spread to another area. There is another organism on this planet that follows the same pattern... a virus. Human beings are a disease, a cancer on this planet, you are a plague, and we... are the cure." - Agent Smith
Excellent story well told. Kudos to David Quammen.
Mother Nature has checks and balances to keep things in check when a population of any one species becomes to populous she drops the hammer. we call this a "Pandemic" We, humans, are long overdo for a culling our technology has keep our culling at bay but the denser our populations gets the more susceptible we become and the worse it will be. This isn't my speculation its fact. I hope my time is done on this earth before the big one hits. Because I don't like to dig and there's gonna be billions of graves to dig
The cure for the human race will be found with ruminants and the I.C.E.emissions. There will be a hidden poison that will only kill on command from the great antenna .It will be taken by the fish caught without a hook.. The cure for the cure will be the holly grail. Only a third will die directly. from poison.
just a guess.
There are 2 cures coming 1000+ yrs apart. The talking snake in the tree of life is lethal.
Where's Robot to comment on this? Shouldn't he be screami ng bloody murder?
An additional thought, we know that bacteria and virus can also "mutate" by transferring genetic material when they are in close proximity. Marburg has under an electron mircroscope some characteristics of rabies. It isn't impossible that some other RNA virus and rabies existed in the same bat, therefore formed Marburg. So, perhaps a rabies vacine could be modified to treat Marburg. Once looking at disease relatives, couldn't immunizations or cures be found.
It should be obvious where the next apocalyptic pandemic will come from.
Diseases and viruses are bred by us for medical, scientific, and bio-warfare. And we will be the catalyst for our own destruction. I just hope our technological sciences can keep up with us. We just may need the infamous artificial intelligent robotics to help the human species to survive in the future.
That is if we as a species don't come to a golden age in understanding, and are able to finally come together and put petty differences aside with religion and race.
Or not, I have always had a fascination with a zombie apocalypse. In some ways this would be beneficial. Most of the world will die but those that remained could finally move on. Drugs and gang bangers would be a thing of the past most of religions would be forgotten and only pertinent skills would remain. We could finally have a world of peace bloom from the remains of the human race.
Pandemics are stopped by the unexciting measures taken to slow them down. This favours strains of the virus which are less lethal and which the body can develop an immunity to before the host is killed. Measures to slow down a pandemic include isolation of all detected carriers and their contacts, vaccination as soon as one is developed and, yes, the spread of the virus around the world in it's less lethal form so that next time around, as far as it is concerned the population is far less than on it's initial foray. The problem with this approach is that the more successful the authorities are in slowing and hence weakening the virus, the more they are accused of over reacting.
HERE IS A NOVEL IDEA...ERADICATE THE CAVES INHABITANTS AND SOLVE A GOOD PORTION OF THIS SCOURGE...AND CONTINUE TO DO SO AT ANY KNOW CAVE DWELLING...
Your shift key is stuck, but yes, eliminating disease vectors is a traditional approach proven effective. This is why yellow fever and malaria afflict few people in the United States, for example.
Pandemics could be easily prevented with the use of nuclear weapons. World powers like China, United States, Soviet Union, Britain and such could stop emerging viruses by essentially sterilizing large areas around emergence sites with fallout, thereby preventing the spread of disease.