When a new virus emerges, it infects organisms that have not developed immunity, or resistance, to the virus. If a new virus infects humans, it may spread easily from person to person before a vaccine can be produced. A disease outbreak that affects large areas of the world and has a high fatality rate is called a pandemic. The disease is spread through infection—for example, by sneezing or coughing—to a great number of people, very quickly.
The 1918 flu pandemic was the most devastating pandemic recorded in world history. This virus infected nearly one-fifth of the world’s population, killing about 50 million people worldwide. It spread mainly along global trade routes and with the movement of soldiers during World War I.
If a new and deadly disease emerges today, a carrier could travel around the world in 24 hours. Several million people travel internationally by plane every year, easily reaching their destinations before they show any symptoms of carrying a disease.
The “Perfect” Virus
Not every virus is well suited to cause massive human casualties. For many viruses, humans represent a dead-end infection because they cannot be passed from human to human. For other viruses, victims die too quickly for the virus to reproduce. Quarantines can contain this type of virus relatively easily.
What characteristics would make an emerging virus likely to cause a pandemic? The virus would need to be adapted to humans as hosts and easily spread through casual contact. Victims would also have to survive infection long enough without symptoms to go about their daily business and infect other people. Finally, the most deadly virus would mutate rapidly, foiling the attempts of scientists to develop a vaccine or a drug that targets it.
Diseases That Jump to New Species
A zoonosis is a disease that can jump between species. A virus that evolves the ability to jump from a nonhuman animal species to humans will spread very quickly in the human body, which has not yet developed defenses. If this virus exchanges genetic material with another human virus, the virus may become capable of spreading from person to person.
The swine flu pandemic was caused by the H1N1 virus that originated in pigs. In 2009, it was estimated that 22 million people were infected with the H1N1 virus! World health officials urged individuals to get vaccinated and educated people on its symptoms. A year later, the swine flu was declared officially contained.
Avian Flu H5N1
Perhaps the most familiar zoonosis is the avian flu virus. Sometimes called the bird flu, this virus normally infects wild birds such as ducks and geese as well as domestic birds such as chickens. Migrating birds can carry it to other continents.
Researchers have been tracking a form of avian flu called H5N1. Like other flu viruses, H5N1 mutates rapidly. Random mutations may or may not help the virus adapt to new host species. However, viruses can mutate in a faster, less random way. If an animal becomes infected with viruses from two different species at the same time, the viruses can exchange genetic information. If this happens, the avian flu can jump the species barrier, becoming a flu virus that can be transmitted from one human to another.
Despite the danger that a new virus represents, no one knows how the virus may mutate or whether it will cause a pandemic. Some of the most important questions include the following:
- How can vaccines be developed quickly enough to stop a disease that can spread in hours or days?
- Can a broad-spectrum antiviral drug be developed that could target more than one flu virus?
- What specific molecular factors allow a virus to jump from one species to another?
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Dissecting a Virus
Scientists have long debated how the genetic material of influenza A viruses, RNA, is likely arranged. In 2005 virologist Yoshihiro Kawaoka and his team of researchers at the University of Wisconsin unraveled the mystery using a technique called electron tomography. Electron tomography is a way to construct a three-dimensional image from a series of electron microscope images taken at different angles. By making slices along flu virus particles that cut them into “top” and “bottom” halves,
researchers found that all influenza A viruses have a total of eight RNA strands. As shown at the right, seven strands form a circle just inside the edge of the virus particle, surrounding an eighth strand in the center.
Based on this similarity in structure, the researchers concluded that all influenza A viruses must share a specific mechanism for packaging their genetic material. This knowledge may make it possible to engineer viruses that can be used to mass produce vaccines to defend against these viruses, which are
responsible for regular seasonal outbreaks as well as the avian flu.
Epidemiologist in Action
In 1993, a mystery disease began to kill people in the southwestern United States. One of the experts that the Centers for Disease Control (CDC) consulted was Dr. Ben Muneta. Dr. Muneta is an epidemiologist, a scientist who studies the causes, transmission, and control of diseases within a population. He works at the Indian Health Service National Epidemiology
Program in Albuquerque, New Mexico.
Dr. Muneta consulted a traditional Navajo healer. From him, Dr. Muneta learned that the disease was associated with extra rainfall, which had caused the pinon trees to produce more nuts than usual. This in turn had led to a population explosion among mice that feed on these nuts.
Using this lead, CDC researchers determined that the disease was caused by hantavirus, a virus spread through the droppings of deer mice. With further research, Dr. Muneta confirmed that some Navajo healers had even predicted the 1993 outbreak.