h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

I currently have swine flu (Day 6).

Apparently I stop being infectious when I stop having symptoms.

As far as I can tell, that means when I have stopped coughing, I can leave the house. That makes sense. Coughing is a good way for a virus to get its host to send it out into the world, to find its next victim.

I understand how I got the virus - it's most likely I breathed in at the wrong moment - but (and this is the question, SExperts), how does it leave me? What happens to it? When I reach the point of being "not infectious", that's from a medical point of view, surely - the virus is still there, in me, isn't it?

If I stop coughing tomorrow - well, hallelujah, I'm cured! - is it still there? Is it in my menstrual blood? In my sweat? (It's certainly *caused* a lot of sweat, more than I thought was possible).

I'm not hysterical about this, or worrying that I've been taken over by aliens.* But I don't understand the end of the viral process, when it leaves its host. Any explanations gratefully received.

Mol

*Well. Actually I have been taken over by aliens, because I have stopped smoking. I didn't want to stop, I didn't choose to stop, I just stopped, on Day 2. I am unbelievably hacked off about this.

The infection ends when your immune system kills off any remaining virus in the body. While you're infected, your body is in a state of balance between 1) the virus infecting new cells, replicating and then leaving the body (via coughing & sneezing); and 2) the immune system killing infected cells before the virus can replicate, generating antibodies to virus particles, raising your body temperature etc. Eventually (hopefully...) the immune system gets the upper hand and manages to kill off all the virus in the body, at which point your symptoms subside and you're free of infection.

Ah! plague! Everybody put a barricade round Mol and don't talk to her in case you get the virus through the keyboard

Sorry don't know where that came from- just went all medieval there. As far as I understood, virus's recede when the body has created antibodies that recognise and destroy any new viruses as they enter the body. Or I may be wrong

Get well soon by the way

Thanks Winnoch - I'm much better now although with a secondary chest infection being treated by antibiotics. Which I *assume* means my GP has taken the view that the virus has run its course. I did have the urge (but not the energy) to daub 'unclean' on the front door for the duration.

Danny - thank you, that was such a clear explanation that I'm almost embarrassed about having asked the question; but I was running a temperature at the time. I was struggling to visualise what was going on and had *completely forgotten* that I have an immune system working all over my body. I was also despairing of ever feeling normal again!

Right, off to top up my fluids and have a nourishing slice of toast ...

Mol

Danny are you sure it's as black and white as that? How does seasonal flu survive from one season to the next if it's not living on humans asymptomatically?

Gotta admit I have a tinge of fear now that we've had word at work that swine flu is advancing through the Uni I work at.

Even though we know it's no worse than normal flu...

...it's still the flu.

And flu is vile.

And Uni's are a nightmare for disease spreading as there are so many people coming in from all sorts of different places.

That's actually quite a tricky question, as no-one really knows why influenza shows a seasonal cycle (and not all strains do, as the hundreds of thousands of cases of swine flu we're experiencing in this year's northern hemisphere summer...) The virus doesn't disappear from the population in the summer, it just moves elsewhere - don't forget that, somewhere in the world, it's always flu season (in the tropics, it's *literally* always flu season - they get to share both the Northern and Southern hemispheres' flu seasons ). As Orcus says, places like Universities, with people mingling from all parts of the world, are great for ensuring that the virus doesn't die out! On top of all that, birds and some animals (pigs, obviously...) act as a reservoir for the virus, and can carry it without getting ill.

Danny, are you saying that seasonal flu varieties ALWAYS need an active infection in a human to keep the species going? Maybe there are subclinical infections that allow the virus to reproduce even though the human may not feel ill.

I suppose there may be subclinical infections, but I've never come across anything about it - I've just done a MedLine search of the scientific literature, and a search for 'influenza + subclinical + infectious' produces only 12 papers between 1975 and 2009. The most recent of these is a mathematical modelling paper of various 'what if' scenarios based on the possibility of subclinical infection.

Even if influenza doesn't explicitly *need* continuous active infection to survive, it almost certainly has it: as I said in my previous post, it is active in the tropics all year round, and travels from the northern to the southern hemisphere on a seasonal cycle (mutating slightly as it goes, so the virus we in the north get back from the south every October is slightly different to the one we gave to the south the previous May!)

Also, I thought it were the case that the influenza virus never goes away in the summer, just the level/rate of infection drops off.... so its still there in the population ready to spring back... err in the autumn winter...