Fact: We contains more DNA that makes viruses than DNA that makes humans.
No, this isn’t talking about Mr Smith’s famous quote about how humans are more like viruses than mammals. 5-8% of all human DNA contains genes that make virus proteins, while only ~1.5% of human DNA contains genes that make human proteins.
This is because viruses like HIV (called retroviruses) shoe-horn their DNA into ours while infecting us. However, sometimes this DNA ‘forgets’ how to leave the genome, and so gets stuck in amongst our DNA.
Occasionally, this will happen in a sperm or an egg, and even more occasionally that sperm or egg will be fertilized and make a whole new human.That child will therefore have that virus DNA in every single cell in their body. Crucially, they will have it in their sperm or eggs, and so if they have have kids they will definitely pass it on.
Ancient viruses have done this millions of times, and so many of them are still kicking around that we are at least 5 times more a virus than we are a human. Luckily, they are all stuck where they are, so the chances of you dying of some million-year-old plague or something are pretty slim.
‘Another one of Mr Shaunak’s Little Bites of Science’
Image under Creative Commons license, by The Wellcome Trust, unedited.