The tiny tagalongs aren't exactly alive by most definitions, but they're not really inanimate either. They've flourished and diversified for billions of years and perhaps even had a hand—or a squishy protein coating—in helping the first complex cellular life come to be. While these microbes have a dizzying array of functions and health effects, the structure of a virus is surprisingly simple.
Some are additionally enveloped in a soft, lipid wrapping. These tiny virus packages are just tens to a few hundreds of nanometers across. This makes them smaller than most bacteria, which can be a small as roughly a tenth the size of a human blood cell.
Such a tiny size means that you can't even spot most viruses with a light microscope. The one exception, a group known as giant viruses , has members with astonishingly large genomes. These mega-viruses are hundreds of times larger than most, with capsids that span roughly to nanometers across and full viral forms that can measure up to nanometers across. Due to their simple structure, viruses cannot move or even reproduce without the help of an unwitting host cell.
But when it finds a host, a virus can multiply and spread rapidly. To identify the correct host, viruses have evolved receptors on their surfaces that match up with those of their ideal target cell, letting the virus get its genetic material inside and hijack its host's cellular machinery to help it reproduce by multiplying the virus' genetic material and proteins.
Using that strategy, the minute marauders have flourished and evolved in step with their hosts. By one estimate , at least , different viruses can infect mammals alone, and even this massive number may be on the low side.
This viral army can cause symptoms as mild as a cough or as deadly as internal bleeding. Some viruses may even cause the runaway cellular growth that is the root of cancer, as is thought to be the case with human papillomavirus and cervical cancer.
Inside their cellular hosts, viruses can create an enormous number of copies and spread the infection to other cells. For example, if you get the flu, your body will be riddled with some hundred trillion viruses in just a few days —more than 10, times the number of people on Earth. How viruses spread from person to person depends on the type. Many hitch a ride in the mist of droplets that flies from your mouth every time you cough or sneeze. A variety of factors can influence how fast these airborne viruses can spread.
Flu, for one, seems to survive longer in cool, dry environments , which may be the source of its common winter spread. But in tropical regions, high humidity seems to help the flu jump from person to person. Other viruses spread most easily through contact with other bodily fluids. For example, Ebola virus spreads from contact with infected blood, feces, or vomit.
Unlike many other viruses, scientists think Ebola cannot spread through the air after people with the virus cough or sneeze. Still other viruses travel through an intermediary, like a mosquito, which then infects people by biting them.
The number of confirmed cases in the U. Even though this particular viral strain only recently emerged as a new human disease, coronaviruses have been around for a very long time. Likewise, Susan Weiss, PhD , a professor of Microbiology at the Perelman School of Medicine, is newly quite busy launching research projects to help respond to the threat of the novel coronavirus — but coronaviruses generally have been a major focus of her research for four decades.
Before , there were only two known coronaviruses that infect humans, both of which are causes of the common cold, but not fatal disease. Weiss noted that the earliest-known coronaviruses also included several viruses that caused disease in other animals including pigs, cows, cats, and birds, so scientists have focused on developing vaccines for those diseases over many years.
Since , when murine coronavirus also called mouse hepatitis virus was first isolated, scientists have studied it in the lab, particularly some of its common strains that infect the liver and brain. Ultimately SARS dissipated within about eight months. Since there have been no more known cases. Over long periods of evolution, you have a virus that comes into a host—a species—and spreads.
One of three things can then happen. One is that the virus can just die out for some reason—everybody can get immune, so the virus can no longer find a host, and it dies out. The second is that the host can die out. Some viruses that we have around with us—like the common cold virus—are like that. There are many viruses like that, such as HIV. This coronavirus probably is, too.
One example is cytomegalovirus, CMV. Many of us are infected with CMV, without consequence. Somebody can have normal vision on Friday and be totally, irreversibly blind on Monday. It varies greatly. You need really close intimate contact, blood transfusion or sexual contact. This coronavirus, by comparison, seems to be relatively stable so that it is able to survive in the environment for hours and maybe a few days, and others are even more stable than that.
Polio virus is stable even in sewage—you pick it up by drinking contaminated water. It passes through the gut, through the stomach, which is almost like pure hydrochloric acid—and the virus is still stable in that. So some are amazingly tough, and some are quite fragile. Taylor McNeil can be reached at taylor. Skip to main content. A Tufts researcher explains the tiny infectious agents that can wreak havoc globally. Here, an image of an isolate from the first U.
The spherical viral particles, colorized blue, contain cross-section through the viral genome, seen as black dots. Photo: CDC.
0コメント