A [Google search][1] reveals many claims that sharing earphones is unhealthy. 

[Marie Claire magazine](http://www.marieclaire.co.uk/news/health/552512/the-real-reason-you-shouldn-t-share-earphones-is-gross.html) provides an example:

> Kelly Reynolds, Ph.D., associate professor of environmental health at the University of Arizona also told Buzzfeed US that 'studies have shown that earbuds cause an 11-fold increase in bacteria in the ears' and 'when you share headphones, you’re doubling the microbial flora in your ears and introducing new bacteria'. [...]

> Bad earphone hygiene can also cause a whole host of nasty ailments too: Middle ear infections, fungus, swimmer's ear, blackheads, pimples to name a few. And Reynolds noted that because our wax also contains strep bacteria, if we have a cut or open wound near to the wax, it could also cause a serious skin infection. 

I'm failing to find any scientific studies regarding this matter.

Picking one claim, does sharing earphones increase the risk of middle ear infections?

  [1]: https://www.google.pt/search?q=sharing%20earphones&biw=1366&bih=635&ei=xriNV_mbMYGba6uvlcgM&sqi=2&start=10&sa=N#q=sharing%20earphones&start=0