Every time a virus gets you sick, your immune system keeps a record. This essentially becomes a kill list that lets your body recognise and readily dispatch of any virus that tries to invade again. Scientists have now created $US25 test blood test that prints out this list — an easy and cheap way to find out every virus that's ever made you sick.
To understand how this test, called VirScan, works, you need to know a bit about human immunity. The immune system responds to a viral infection by making antibodies, proteins that then bind to viral proteins and render them useless. Small amounts of these antibodies keep circulating in your blood even after you recover. They lie in wait for the next time you encounter that virus.
Existing tests for viruses — say for HIV or hepatitis C — in fact already look for these antibodies. What makes VirScan different is that it can look for antibodies matching virtually every virus known to infect humans at once. That's 1000 strains from 206 species.
The team behind VirScan had to do some clever bioengineering to make that happen. First, they took viral DNA sequences downloaded from a virus database and put them inside the genomes of bacteriophages, which are a special class of viruses that only infect bacteria. This made the hybrid bacteriophages display proteins from a totally different virus, say the flu virus, on their surfaces. When a blood sample was mixed with the bacteriophages, any antibodies in the blood would stick to a bacteriophage displaying a corresponding virus's proteins. The last step was simply sequencing those bacteriophages to read out a person's viral history.
From the 569 people the team has screened so far, the team found that people have the antibodies for an average of 10 viruses. VirScan, if it's perfected for routine use, could be used to look for common factors in mysterious illnesses such as chronic fatigue syndrome.