How 'infected' are we? – Viruses

by Vincent Racaniello, PhD

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    00:00 that's how big 10 to the 16th is. Each of us is infected with at least a dozen viruses at any given time, probably more. When I talk to audiences about viruses, one of the first questions I ask is, please raise your hand if you're infected or has had a virus infection and only about 10% of the people answer. But the fact is that we're all infected, always.

    00:21 I'm infected now and so are you and there is no getting around it. We have at least a dozen herpes viruses in us, and on this slide is a picture of a herpes virus. It's a large DNA containing virus. And we acquire these infections when we are quite young, and we keep them for our whole lives. Herpes simplex viruses, varicella zoster, human cytomegalovirus, Epstein-Barr virus, and a variety of other herpes viruses, they all infect us, they stay with us. Once we're infected, it's for life and we think that some of these viruses may in fact be beneficial for us. It's true that from time to time they do cause disease, but the fact that they're present throughout our life we think its a clue that they may be helping us.

    01:08 An interesting point to look at in terms of how viruses help us is in our genome, in our DNA. The human genome is about 3.2 billion bases in length and we know its sequence.

    01:21 It has been sequenced in many different individuals, so we know how it's made up. And let's take a look at what makes up our genome. A fraction of our genome is made of repeated elements called sines and lines, in fact the part of our genome that codes for protein is very small, as you can see by this slice of the pie. There are intervening sequences which don't encode proteins, which are a big fraction of our genome. Miscellaneous unique sequences and heterochromatin. There are duplications of DNA. There are sequence repeats. And then there are DNA transposons which are quite interesting. These are DNAs that hop around.

    02:02 But perhaps for our discussion, the most relevant point are the LTR retrotransposons. These are viral sequences that are part of our genetic information. You can see they make up a good fraction of the DNA, they are about 8% of our genome. They are result of infections that happen many, many years ago, millions of years ago and have remained with us ever since. And in fact some of these genes encode proteins that help us. So a virus gave us a gene many years ago that is useful to us today, so we're really truly viral.

    02:40 All of these viruses that we inhale and eat and touch on a daily basis, most of the time they don't hurt us. And that is because we have a great immune system which is diagrammed here. We have an immune system that can fight invaders such as viruses and other pathogens and get rid of them before they cause a problem. It's only when your immune system is down that you have a problem with a virus infection. So for example if you are immunosuppressed, if you have a virus infection that is immunosuppressive like HIV, other viruses will then cause problems.

    03:17 If you take drugs to immunosuppress you, let's say you've had an organ transplant, you get drugs so that your body does not reject the organ, your immune system is now compromised.

    03:27 You can't fight off those smallest virus infection. So we have our immune system to thank for this wonderful balance between viruses being in us and not doing anything and viruses causing disease.

    03:42 I would like to make the case that we already have evidence that many viruses are good and here is one example of that. There is a small DNA virus diagrammed here called polyomavirus, which seems to infect everyone, starting at the young age throughout their lives. And as far as we can tell, unless you're immunosuppressed, these viruses don't hurt you at all. We think they're probably beneficial, but we don't have evidence for that yet. But the interesting thing about these polyomaviruses is that because they are transmitted within families, they are markers of human migration. So for example, we know that humans originated in Africa and from Africa moved to Europe, to Asia and then to the Americas, and that's shown by the dotted line here. This is a migration pattern that you can deduce by looking at the sequences of human DNA. If you look at the polyomaviruses that people carry, you get the same conclusion, that humans originated in Africa, they moved to Europe and we even have more detail when we look at the polyomavirus tracing, we can see that they went down into Australia and Australasia and then over to North and South America. So these are uses of a beneficial virus and they are probably more that we remain to discover as well.

    About the Lecture

    The lecture How 'infected' are we? – Viruses by Vincent Racaniello, PhD is from the course Microbiology: Introduction.

    Included Quiz Questions

    1. LTR retrotransposons
    2. Introns
    3. SINES
    4. LINES
    1. Our immune system is effective at limiting viral disease.
    2. These viruses do not replicate well.
    3. None of the given answers are correct.
    4. The viruses interfere with each other.
    1. 3.2 billion
    2. 2.3 billion
    3. 2.4 billion
    4. 4.2 billion
    5. 2.6 billion
    1. LTR retrotransposons
    2. Introns
    3. Segmental duplications
    4. SINEs
    5. LINEs
    1. Migratory patterns
    2. Immunosuppression
    3. Hereditarily malignancy
    4. Dietary choices
    5. Occupation

    Author of lecture How 'infected' are we? – Viruses

     Vincent Racaniello, PhD

    Vincent Racaniello, PhD

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