Even before we became aware of emerging diseases like HIV and Ebola, there seemed to be plenty of viruses around. It felt like childhood was a blur of various illnesses—and these were the ones we hadn't been vaccinated for. So just how many types of viruses does a typical human get exposed to?

Ten, if a study in this week's issue of Science is to be believed. The study introduces a new way of getting a global history of all the viruses a person's immune system has had the pleasure of knowing. The technique has some significant limitations, but it still has the potential to provide new perspectives on how the human immune system functions.

First, the technique, which its creators are calling "VirScan." It relies on the fact that, after a person's immune system mounts an attack against a pathogen, a small collection of B cells, called memory B cells, continue to produce antibodies that recognize the invader. These allow the immune system to mobilize rapidly if the same pathogen is ever encountered again. But the memory B cells also allow us to study the antibodies they produce.

So, how do you find what antibodies are present? The authors simply searched a major sequence database for every protein known to come from a virus that targets humans. The split these proteins up into overlapping chunks of amino acids, and then designed DNA sequences that encode each chunk. These DNA sequences were then placed into the genomes of viruses that infect bacteria, which are called phages.

Collectively, the resulting population of engineered phages displayed pieces of every known human viruses on their surfaces. As a result, if a human antibody recognized a known human virus, they'd also be likely to stick to some of the phages.

The authors then got volunteers to donate a small volume of blood plasma, which contains antibodies that would recognize previous pathogens. As a result, the antibodies also recognized some of the engineered phages, and could be used to pull them out of the full library of phages. From there, you just sequence all the phages (which is easy to do in a large batch) and you know which human viruses the antibodies recognize. Therefore, you know which infections the human donor has had in the past.

The authors performed this test with donors from around the world, and found a number of patterns. They were able to detect antibodies to an average of 10 viral species per donor, but the numbers varied widely—two donors had been exposed to at least 85 different viruses. Children, not surprisingly, were exposed to fewer viruses on average; US residents also tended to have lower infection rates.

But, while it provides a clearer picture of the world's human-targeting viruses, that picture's also probably wrong. The authors take a lot of steps to avoid false positives but, in doing so, they probably get rid of some genuine signals. "The frequency at which we detect influenza (53.4 percent) and poliovirus (33.7 percent) is lower than expected," the authors write, "given that the majority of the population has been exposed to or vaccinated against these viruses."

There are several reasons why the numbers are so low. One is that the proteins on the surface of a virus have to fold into very specific shapes to function. In many cases, antibodies recognize these three-dimensional shapes. But the phages used here only contain small fragments of the full protein, so they probably won't fold properly, which means the antibodies won't recognize them. In many cases, proteins produced in human cells get modified by having other chemicals (often sugars) attached; these won't happen in the bacteria used to produce the phages. Again, this may leave the antibodies unable to recognize the final product.

The authors test out various analytic approaches to detecting true positives. But there are a number of viruses they can't detect at all, and other viruses where the numbers still seem low. It's also worth nothing that this only works with viruses we know about and have done DNA sequencing on, so there may be a lot of things that aren't even looked at.

VirScan is a step forward, and there's several questions it can address effectively right now. It also has lots of potential for further improvements and refinements. It just isn't anywhere close to the point where it can actually tell you every virus you've ever been infected with, which is what some of the reports in the press have indicated.

Science, 2015. DOI: 10.1126/science.aaa0698 (About DOIs).

Listing image by Flickr user El Alvi