Colds, the flu, and other, deadlier diseases can spread very quickly.
It could be worse. Earlier this year, many people died in China and other countries from a disease called SARS (Severe Acute Respiratory Syndrome). Some schools and hospitals in Toronto, Canada, and elsewhere had to shut down for days to help keep the disease from spreading.
And if you've been following the news lately, you may have heard about the dangers of not only SARS but also monkeypox, mad cow disease, and the West Nile virus. Animals have died. People have gotten sick. Sometimes, panic has set in.
The culprits responsible for most of these ailments are tiny, tiny organisms called viruses. Unlike people, animals, and plants, viruses are not made up of cells, but they do contain some of the building blocks of cells. The most important pieces are the molecules DNA and RNA: sets of instructions that tell cells how to make more cells of the same kind. A virus carries instructions for making more viruses.
When certain viruses invade your body's cells, they can cause your body to react, and you get sick. Your body gets so busy making new copies of a virus that it can't do what it's supposed to do. And when viruses spread easily from person to person or from animal to person, a disease epidemic may occur.
Respect for microbes
In the midst of the SARS outbreak last spring, I came down with a horrible cold that kept getting worse. Many of my symptoms sounded like SARS. My lungs hurt. I had a sharp cough. I felt feverish. Terrified, I rushed to the doctor. When he told me I had bronchitis, I was relieved. I still felt miserable, but my fear of having SARS had far outweighed any suffering I felt from bronchitis.
But we don't have to be scared all the time. By arming ourselves with knowledge and adopting a few good habits, experts say, people can stay healthy and strong. We might even learn a few things about the invisible world around us.
The first lesson is respect, says Amy Vollmer, a microbiologist at Swarthmore College in Swarthmore, Penn.
"We survive on this planet not because we're superior," Vollmer says. Bacteria and viruses far outnumber us, and the tiny organisms have been here a lot longer than we have.
"Microbes have been on the planet for 4 billion years. Humans have been here for a million or so," Vollmer says. "They were here first. We have developed and survive around them."
Most microbes don't affect us at all. Some actually help keep us healthy. But the ones that get our attention are the ones that make us sick, especially if they can easily jump from one person to another.
Our immune systems help protect us against such microscopic invaders. These systems are like soccer players: They get better with practice.
Vaccines such as flu shots help your body gear up to fight viruses.
When an infectious pathogen attacks your body for the first time, you might get really sick for about a week, while your immune system gears up to fight back. The next time you face the same virus, though, your body remembers what to do. Your immune system takes only a few days to kick into gear. You might not even feel any symptoms.
Vaccines such as flu shots take advantage of this gearing up. They expose your body to a little bit of a disease, which gives your immune system a dress rehearsal for fighting the invader in case of a more serious attack later on.
Identifying the culprit
In recent years, infectious diseases such as SARS and monkeypox have become more common all over the world, says parasitologist Peter Daszak of the Consortium for Conservation Medicine in Palisades, N.Y. The trend is probably our own fault, he says.
"Diseases evolved to be very good at moving from one population to another," Daszak says. "It's what they do best. What we're doing now is creating ways for them to move like they've never done before."
Many people get sick when they take trips to exotic places where they encounter unfamiliar microbes. If it takes a few days before infected travelers show symptoms, they can spread a disease without knowing it. Some people may even carry and spread a virus without ever getting sick themselves.
When a new epidemic first shows up, scientists start looking for its source. They conduct interviews to uncover patterns about where patients have been or what they've eaten. At the same time, doctors keep infected people in isolation to try to stop the disease from spreading. During the SARS outbreak, people in Asia wore surgical masks in public so they wouldn't inhale the virus.
Next, scientists race to identify the culprit by extracting it from an infected person and testing whether it can cause an infection. When researchers are sure of the cause, biochemical analysis begins.
Investigations quickly showed SARS to be caused by a coronavirus, one of many different families of viruses. Analyses of the monkeypox virus revealed that monkeypox is related to a horrible disease called smallpox. This kind of information can help scientists narrow their search for the right kind of drugs or vaccines to prevent future outbreaks.
As scientists learn more about disease epidemics, animals turn out to be a vital link.
SARS, for example, started out as a disease in palm civets. A palm civet is a badger-like mammal with spotted fur and a long tail that lives in southern Asia and tropical Africa. Now, researchers have found that cats and ferrets can carry the SARS virus, but no one is sure whether they can spread it to people.
Mosquitoes transmit the West Nile virus. Monkeypox first spread to people in the midwestern United States through pet prairie dogs. The disease had previously appeared only in western Africa.
Yet, animals might be as much a casualty as a cause of epidemics, Daszak says. Diseases may be spreading more often from animals to people simply because people are handling animals without being careful enough, he says.
"We shouldn't really blame animals," Daszak says. "We should blame humans that change animal habitats, humans that trade animals and move them from one place to another, and humans that destroy forests and invade animal homes."
By protecting animals, he says, we also protect ourselves.
For now, all the talk about disease epidemics doesn't mean you need to hide inside all day long. "The best way to stay healthy with all these diseases is to know about them," Daszak says. "It's really fascinating rather than scary."
Washing your hands often and carefully can help keep infections from spreading.
Simple precautions can make a big difference. Avoid mosquitoes to protect yourself from the West Nile virus. To prevent monkeypox, don't buy exotic animals. If you do want an exotic pet, have a doctor screen it for diseases first.
It also really helps if you wash your hands a lot. And, if you're sick, you should stay away from school and other people.
Most important of all, Vollmer says, is to take care of yourself. By eating well and sleeping enough, your immune system will stay nice and strong.
"If you can learn that early and keep it up as you get older," she says, "you can live a long and healthy life."