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A new blood test can find just about every virus you ever caught — in a single drop of blood.

It’s not just an interesting historical record — doctors hope the test might be used to find out whether viruses might cause a range of chronic diseases such as diabetes or heart disease, and to see whether infections early in life can affect your immunity later.

"VirScan is a little like looking back in time: using this method, we can take a tiny drop of blood and determine what viruses a person has been infected with over the course of many years," said Stephen Elledge, a Howard Hughes Medical Institute researcher at Brigham and Women’s Hospital in Boston, who led the study.

"VirScan is a little like looking back in time."

“Right now, a physician needs to guess what virus might be at play and individually test for it. With VirScan, we can look for virtually all viruses, even rare ones, with a single test."

The test looks for the body’s immune response to viruses. It finds antibodies that keep the viruses from coming back, which can stay circulating in the blood for years.

Elledge thinks the test will cost about $25 to run.

There are thousands of viruses, from rhinoviruses and adenoviruses that cause the common cold, to the human immunodeficiency virus that causes AIDS. A total of 206 different species of viruses are known to infect people, and each species has various strains.

Most blood tests are designed to look for one particular virus at a time. Elledge and his colleagues wanted to design a test that would find all the viruses — a collection called the virome.

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They developed the test using bacteriophages, which are viruses that attack bacteria. They engineered different bacteriophage to make a little piece of protein from more than 1,000 known viruses.

The test goes into a drop of blood, which is enough to carry all the antibodies trained to recognize a virus that’s infected someone — or those activated by a vaccine.

Writing in the journal Science, the team said they tested blood samples from 569 people from the United States, Peru, South Africa and Thailand.

On average, people had antibodies against 10 species of virus. There were a few people who’d been infected with many different viruses — five people had antibodies against 62 species, and two had been infected with 84 different species.

It’s well known that viruses can cause disease. The human papilloma virus (HPV), for instance, causes cervical cancer, as well as head and neck cancer and cancers of the genitals. Epstein-Barr virus, which infected 88 percent of the volunteers, is linked to lymphoma and is suspected of causing some cases of stomach cancer. It’s also been linked with multiple sclerosis.

Enterovirus D-68, a distant relative of polio, concerned doctors when it made an unusual resurgence last year and appeared to cause a mysterious polio-like syndrome in a few children.

Having a test that can look for all sorts of viral infections at once can help researchers tighten up these links, and also help them find possible other links between viruses and long term health.

“Right now, a physician needs to guess what virus might be at play and individually test for it."

It’s not perfect. Elledge said some viruses didn’t turn up as often as they expected.

“For example, the frequency at which we detect influenza (53.4 percent) and poliovirus (33.7 percent) is lower than expected given that the majority of the population has been exposed to or vaccinated against these viruses,” they wrote.

It also only found that about 24 percent of the people tested had antibodies to chickenpox, also called varicella, which is far fewer than expected.

But when they tried the test against people who knew they had HIV and hepatitis C, the test was 95 to 100 percent accurate. “We didn't falsely identify people who were negative,” Elledge added. “That gave us confidence that we could detect other viruses, and when we did see them we would know they were real.”

Rare viruses were appropriately, well, rare.

"Although we detected antibody responses to rare and highly virulent viruses such as Marburg and bat lyssavirus, they were found in less than 0.4 percent of the population," they wrote.

Elledge’s team says the test could easily be expanded to include other human pathogens such as bacteria, fungi, and protozoa. The test might also be used to find out if there are consequences of being infected with two or more particular viruses.