[Donnah (OP)]

If someone else uses my brush and then I use it, my hair becomes oily within hours, even if it's freshly washed.  Why is that?

[Geezer]

If someone else uses my brush and then I use it, my hair becomes oily within hours, even if it's freshly washed.  Why is that?

Perhaps the other individual is too unctuous?

[RD]

If someone else uses my brush and then I use it, my hair becomes oily within hours, even if it's freshly washed.  Why is that?

You are inoculating your scalp with the other person's skin bacteria, which could trigger extra oil production within a few hours. Sebum [oil] produced by the scalp / skin is a bactericide ...

Tear ducts, sebaceous glands, ears, nose, lungs, mouth and digestive tract all secrete antimicrobial substances

http://www.microbiologytext.com/index.php?module=book&func=displayarticle&art_id=384

[Donnah (OP)]

Goodness Geezer, I had to look unctuous up in the dictionary.  It doesn't matter how squeaky clean the other people using my brush are, it elicits the same response.  Even when I don't know that the brush was borrowed.

RD, what you say makes sense, an immune response is triggered.  I'll try brushing the animals with my brush and see if it has the same effect.

[RD]

... I'll try brushing the animals with my brush and see if it has the same effect.

Transmission of infection from hosts who are members of the same species is more likely than if the hosts are different: it may difficult or impossible for some infections to cross a species boundary.

IMO everybody, including the animals, should have their own brush: why risk some horrible skin condition in human or pets, e.g. , (and the cost of treating it), for want of a few cheap brushes.

[Donnah (OP)]

I've hugged, kissed and cuddled animals all my life and never had a problem other than a few worms, which we all get anyway from things like doorknobs and keyboards.  I heard on the news that they are giving children pills made from dirt so they can build a decent immune system.

I agree with your caution if the people or pets have a contagious health issue.