This month, a pair of viruses seized the headlines. First came a hantavirus outbreak aboard a cruise ship, which caused as many as 13 infections, three of which were fatal. Then an Ebola outbreak flared in Africa, so far leading to more than 900 infections and 220 deaths.
In both cases, the news has been not only frightening but also confusing, even to scientists. The hantaviruses didn’t seem to be acting like hantaviruses, and the Ebola viruses weren’t behaving like Ebola viruses.
Hantaviruses are carried by rodents and other animals, and typically infect people who inhale dried animal urine and saliva. But aboard the cruise ship M.V. Hondius, hantaviruses were moving from person to person.
As for the African outbreak, scientists have made huge strides in fighting Ebola in recent years. They’ve created vaccines that can slow the spread of the disease and antiviral drugs that can cure infections.
But these treatments are probably going to be weak or useless. This is a very different Ebola virus.
What gives? There is a vast diversity of viruses, but we employ a limited vocabulary to talk about them. It would be just as confusing to treat blue whales like fruit bats and Siberian tigers, simply on the grounds that they’re all mammals.
Jens Kuhn, a virologist who serves on the International Committee on Taxonomy of Viruses, said that the recent outbreaks point to yawning gaps in our understanding of the so-called virosphere, the millions — perhaps even trillions — of virus species thriving around us.
“These are case-use examples of why taxonomy is important,” he said. “Is something the same, or is it different? Well, if it’s different, then stuff we know about the other thing will not work on it.”
Ebola viruses got their names from the site of one of the first documented outbreaks in 1976: the Ebola River, in what was then Zaire, now the Democratic Republic of Congo. When scientists examined blood from the victims, they isolated long, snakelike viruses distinct from any previously known.
But 1976 saw another outbreak that also caused deadly, bloody fevers — this one hundreds of miles to the east, in what was then Sudan, now South Sudan. The infected also harbored snakelike viruses.
When scientists compared the viral genes, however, they found a striking number of differences. In later years, Ebola outbreaks occurred dozens of times, and in most cases the viruses resembled either the type first seen in Zaire or the type seen in Sudan.
Eventually, Dr. Kuhn and his colleagues formally recognized the two kinds of viruses as two distinct species. And, as taxonomists do in such cases, they gave each species a Latin name: Orthoebolavirus zairense and Orthoebolavirus sudanense.
But in the 50 years since the first Ebola outbreak, scientists have found other relatives of these viruses. In 2007, for example, 149 people in the Bundibugyo District of Uganda came down with hemorrhagic fevers, and 37 died.
The virus they harbored was, genetically speaking, over 30 percent different from the viruses isolated in Zaire and Sudan — a new species, known now as Orthoebolavirus bundibugyoense.
The Bundibugyo virus caused a second small outbreak in 2012 before exploding back on the scene this month. The vaccines and the drugs that were developed for the Zaire species don’t work against the Bundibugyo virus, which belongs to a different evolutionary lineage. That’s one reason the new outbreak has public health experts so worried.
Hantaviruses also got their names from a river: the Hantan River, which flows through North and South Korea. It’s in a region where a mysterious kidney disease struck people every year. In 1978, researchers isolated the cause: a virus carried by striped field mice.
Since then, scientists have discovered hantaviruses lurking in rodents and other mammals around the world. Some of them also cause kidney damage, while others strike the heart and the lungs.
The actor Gene Hackman’s wife, Betsy Arakawa, died at their New Mexico home last year after being infected with a type of hantavirus called Sin Nombre. Diagnosed earlier with Alzheimer’s, Mr. Hackman died days afterward.
As hantaviruses have adapted to rodents and other mammals across much of the world, they have evolved an enormous diversity — Dr. Hoeg and his colleagues recognize 38 species in the genus Orthohantavirus. (The Ebola genus, by contrast, includes only six species.)
Each species in turn may harbor a lot of diversity. As viruses replicate, strains can pick up new mutations that can drastically change their biology.
The outbreak on M.V. Hondius this spring was caused by a species called Orthohantavirus andesense, carried by a number of rodents in South America. But there are four strains of this species; the outbreak was caused by one called Andes virus.
Unlike the other three strains — and unlike the 37 other species of hantaviruses — Andes virus can spread directly from one person to another.
“It seems like there are some mutations that under certain circumstances can make Andes virus person-to-person transmissible,” Dr. Kuhn said. No one knows what those mutations are.
Dr. Kuhn suspects that the other strains related to Andes virus are lurking in rodents and share this ability to spread among people. After the M.V. Hondius outbreak, he predicts, scientists in Argentina and Chile will “go into their freezers with all the samples and sequence the crap out of everything and figure out — what are all these things?”
As for Ebola, Dr. Kuhn expects more unpleasant surprises. He points to Orthoebolavirus taiense, also known as Taï Forest virus.
The first and last time anyone saw this species was in 1994, when it infected a scientist dissecting a dead chimpanzee. She developed Ebola symptoms but eventually recovered.
“I’m sure it’s still out there, but nobody focuses on it because it caused only one case,” Dr. Kuhn said. “I think that’s a big mistake.”
Other Ebola-like species that have yet to be discovered and named may be lurking in African animals. The classification system that Dr. Kuhn has helped create will hopefully make it an easier process.
He doesn’t expect people to learn to rattle off Orthohantavirus andesense and other Latin names in casual conversation. But instead of referring to the cause of the outbreak in Africa as the Ebola virus, he suggests calling it Bundibugyo virus. (It’s pronounced boon-dee-boo-joh.)
“The moment you mix up Bundibugyo virus and Ebola virus, the impression will be, ‘Oh, we’ve got something for that,’” Dr. Kuhn said. “But we don’t.”
