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The human body is a rich environment for microbes, replete with proteins, fats and carbohydrates. Many viruses have figured out how to peacefully thrive in it without making us sick.

Viruses need to invade host cells to reproduce, and they are adept at exploiting all the options in our body. A dozen years ago inexpensive genome sequencing led us to discover plentiful viruses in the mouth and gut. By or so scientists located viruses on the skin and in the respiratory tract, blood and urine.

Most recently, we have found them in even more surprising places. In September , for example, Chandrabali Ghose and our colleagues and I published details about viruses that we discovered in the cerebrospinal fluid of adults who were undergoing testing for various conditions. The viruses belonged to several different families and were not associated with any known disease. We also found the same viruses in blood plasma, joint fluid and breast milk.

Scientists knew that a few rare, infectious viruses, notably herpes, could sneak into cerebrospinal fluid, but finding random viruses that seemed to be mere bystanders was a surprise.

The central nervous system, which is supposed to be a sterile environment, is colonized by a somewhat diverse viral community. It appears that our viromes begin to accumulate when we are born. Studies reveal a high diversity of viruses in the infant gut shortly after birth, suggesting that they probably come from the babies' mothers, some ingested from breast milk.

Some of these viruses decline in number as infants grow to weeks or months old; others enter their bodies from the air, water, food and other people. These viruses grow in number and diversity, infecting cells where they will persist for years. Infant viromes are unstable, whereas adult viromes are relatively stable. Anelloviruses, a family of different species, are present in almost everyone as we get older. This mirrors what we observe for bacteria as well. Many of the viruses living inside us do not target our cells.

Instead they look for the bacteria in our microbiomes. Known as bacteriophages , or phages, these viruses sneak inside bacterial cells, use the machinery there to make copies of themselves, then often burst out to infect more bacteria, killing their host cells in the process. Bacteriophages are nearly ubiquitous in nature. If you look hard enough, you will find them in soil, in any source of water from the ocean to your tap at home, and in extreme environments such as acid mines, the Arctic and hot springs.

You will even find them floating in the air. They persist in all these places because they are hunting the bacteria that live in all these places. We humans are just another hunting ground. In Sophie Nguyen and Jeremy Barr, then at San Diego State University, demonstrated that many phages get to their final locations in the body by crossing through mucosal membranes.

In laboratory experiments, phages worked through membranes that line the intestine, lung, liver, kidney, even the brain. But when they randomly cross into a place such as the central nervous system, where there are few bacteria to be hosts, they may have no way to replicate and may ultimately perish. The virome can vary greatly from one part of the body to another.

When Ghose and I looked for viruses in unexpected places, we also determined that viruses in the mouth are different from viruses in the gut, which are different from viruses in urine or in blood. We knew this was the case for bacteria, but early on we did not have enough data for viruses.

Although it is not hard to find volunteers who will spit in a cup, it is hard to get them to provide stool or blood samples and to persuade universities to sign off on obtaining and processing these samples. When we do have the goods, we must filter out the bacteria, leaving tiny bits of viral material we can examine under a microscope and insert into a machine that sequences the nucleic acids that encode the genes that are present.

Still, researchers have done enough of this work now to be able to tell what part of the body they are examining just by noting the viruses present. My colleague Melissa Ly of the University of California, San Diego, and I have also shown that by comparing the viromes of unrelated people we can determine if any of them live together.

Although different people can have significantly different viromes, people who cohabitate appear to share about 25 percent of the viruses in their viromes. Viruses can be transmitted from one household member to the other not just through typical contagious means such as coughing but also through casual contact and sharing sinks, toilets, desks and food.

Although we have only studied small numbers of people, the data show that nonromantic roommates share a similar percentage of viruses as romantic roommates do. Intimate contact seems to make little difference; just living in the same space is enough.

The puzzle is tricky, however. Shira Abeles, also at U. San Diego, has identified big differences in the oral viromes of men and women; hormones could be the reason, but no one has demonstrated such a connection. We do know that viromes can vary considerably with geographic populations. For example, there is less diversity in the viromes of individuals in Western countries than there is among individuals in non-Western countries. These differences may be related to both diet and environment.

Many viruses in our virome infect bacteria, but a smaller proportion infect cells in our tissues directly. These viruses may be in the minority because our immune system suppresses them. Iwijn De Vlaminck, then at Stanford University, demonstrated that when a person's immune system is strongly challenged—for example, when someone has received an organ transplant and must take immunosuppressing drugs to avoid rejecting the organ—the presence of certain viruses increases dramatically.

In these cases, we see a rise in both viruses known to cause disease and those that do not. This observation suggests that under normal circumstances our immune system keeps the virome in check, but when immunity is hampered, viruses can multiply readily. The most common are a secondary bacterial pneumonia, or bacteremia a rise of bacteria in the bloodstream , involving organisms such as Staphylococcus aureus and Streptococcus pneumoniae.

So they must have a living cell in order to survive and replicate. Antibiotics are not effective against viruses, but vaccines are, as well as some antivirals. Bettie J. Graham, Ph. Featured Content. Introduction to Genomics. The main concern with new variants is the unpredictability of their impact. The main symptoms of COVID are dry cough , fatigue , and fever, but there are many possible symptoms.

Anyone who has symptoms should seek a test. It is also important to self-isolate until 10 days after symptoms appear and when no fever has been present for 24 hours. If a person has difficulty breathing , they should seek emergency medical attention. However, special T cells, known as cytotoxic T cells, can recognize cells that contain viruses, and release substances that kill those cells.

Some viruses can escape detection by cytotoxic T cells, but other immune cells — natural killer cells — can cause the cell containing the virus to die. In addition, body cells that contain a virus emit proteins called interferons, which warn other cells that a virus is present. This gives healthy cells a chance to defend themselves by changing the molecular makeup of their surface. Antibodies can also help fight a virus before it enters a cell.

They do this by neutralizing or damaging the virus or by changing its features so that it can no longer enter healthy cells. Antibiotics treat bacterial infections , but they cannot treat a viral infection. People will need either a vaccination to prevent infection, or antiviral drugs to treat any symptoms.

Sometimes, the only option is symptom relief. In recent decades, scientists have developed antiviral drugs, largely in response to the AIDS pandemic. These drugs do not destroy the virus, but they slow or prevent its development. With antiviral treatment for HIV, for example, the level of virus in the body can become so low that tests cannot detect it.

At this point, it becomes untransmittable, which means that a person cannot pass the virus on to another person. Antivirals are also available to treat infection with HSV, hepatitis B , hepatitis C , influenza, shingles, and chickenpox. Tamiflu is an example of an antiviral drug. People can use it to manage influenza. Some vaccines have succeeded in eliminating diseases such as smallpox, which experts believe has been around for at least 3, years.

Here are some ways a virus vaccination may work:. Currently, vaccinations exist for polio, measles, mumps, rubella, COVID, and various forms of the flu, among other conditions. Vaccination can dramatically reduce the likelihood of becoming seriously ill due to a virus, as well as the risk of passing a virus on to others. If there is an outbreak, it usually affects people who have not had the vaccine. As a result, the risk of disease increases dramatically, and an outbreak can occur.

Health experts encourage people to have this vaccine to protect themselves and others. Viruses are biological entities that are present in all living beings. Some are harmless, while others can cause a range of diseases, from the common cold to Ebola. Seeking protection from potentially hazardous viruses — for example, through vaccinations — can help prevent serious illness. As the coronavirus outbreak continues, a host of misconceptions and half-truths surround it.

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