[Brian Jacobs]

Brian Jacobs  asked the Naked Scientists:
   
Dear Chris,
 
It is common cause that there are a number of diseases which can be transmitted from one individual to another because of unprotected sex.
 
My question is whether the diseases that can and are transmitted are restricted to those that are mostly publicised or could the flu be transmitted from one person to the other through unprotected sex?
 
Thanks,
 
Brian. H. Jacobs

What do you think?

[FuzzyUK]

Brian Jacobs  asked the Naked Scientists:
   
Dear Chris,
 
It is common cause that there are a number of diseases which can be transmitted from one individual to another because of unprotected sex.
 
My question is whether the diseases that can and are transmitted are restricted to those that are mostly publicised or could the flu be transmitted from one person to the other through unprotected sex?

So long as your condom doesn't fall off your nose during intercourse, your partner should not catch the flu. Try not to sneeze at the point of orgasm.

[Variola]

Brian Jacobs  asked the Naked Scientists:
   
Dear Chris,
 
It is common cause that there are a number of diseases which can be transmitted from one individual to another because of unprotected sex.
 
My question is whether the diseases that can and are transmitted are restricted to those that are mostly publicised or could the flu be transmitted from one person to the other through unprotected sex?
 
Thanks,
 
Brian. H. Jacobs

What do you think?

I am not 100% on this, but to my knowledge the 'flu virus is not able to survive in the mucosal environment of the reproductive tract, it is not it's natural habitat and it does not infect those cells.
It is designed to live happily in the respiratory tract and is spread by sneezing, coughing and close contact with an infected person. Given that sex usually requires people to be pretty close it could be passed on easily, but not necessarily sexually! 

[wolfekeeper]

If your idea of sex involves not kissing then you may not catch it.

In the real world of non commercial sex, that's not the way it usually goes.

[chris]

In humans influenza (flu) is chiefly a respiratory infection spread by aerosolised liquid droplets that contain viral particles. When these contact the respiratory surfaces, and the eyes, then they can lead to infection. Close personal contact between an infected and a susceptible individual, regardless of what sort of activity you indulge in, can therefore be regarded as a risk factor for catching the infection.

[MayoFlyFarmer]

Chris, your answer reflects my understanding of influenza transmission, but I've never completely understood why that is the case.  What makes a mucus membrane on the eye and different than a mucus membrane on the genitals as far as a suitable site of entry?  Is it simply a matter of physical proximity to the respiratory tract?

[chris]

There are two things to consider here:

1) The eyes are connected to the nose via the nasolacrimal duct, whose normal job it is to drain tears into the nose. Virus particles landing on the eyes can therefore be carried to the respiratory surfaces, which they can readily infect, via the tear film.

2) The cells in the eye are susceptible to infection by flu and a range of other viruses. H7N7 is one example of a strain that had been noted to do this and cause conjunctivitis. From here the virus can spread to the respiratory surfaces (as per 1 above) and also shed from the body in tears. There is therefore an evolutionary pressure on the virus to replicate in the cells of the conjunctivae. But because the genital mucous membranes are less likely to be a strong source of transmission, there is less selective pressure on viruses to optimise their replication for these cells, so although infection is possible, it's probably limited or inefficient in these other sites.

[MayoFlyFarmer]

Good points.  I should have considered the connectivity between the eyes and the nasal passages (and therefore respiratory system).