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

Marie Claire magazine 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?

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

Marie Claire magazine 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?

A quick google search readily reveals that sharing earphones is unhealthy and not recommended. Despite this being intuitive, most of the sources are untrustworthy online newspapers and I'm failing to find any actual studies (from reputed science journals or otherwise) regarding this matter. So...

Should one refrain from sharing earphones? Why?

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

Marie Claire magazine 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?