What is "not nice" to discuss is the possibility of contracting HPV from an open-mouthed kiss while you are just doing your job in your line of work. Mouth closed may avoid or reduce this risk. HPV is mainly known as a venereal disease virus that causes venereal warts and can lead to cancer of the cervix in women though it can lead to other cancers and conditions.
I thought that was pretty obvious and didn't need to be spelled out.
Interesting that you say pneumonia is not contagious, but I think that is not quite accurate. You may be thinking of it as being just a "condition" of the lung but the website below says it is an "infection" of the lungs caused by virus or bacteria which can be contagious.
If you pick someone to date you have a choice; it should be voluntary. You don't usually have the luxury of choosing with whom you work because the actor doesn't generally do the casting, plus an understudy or new cast member can appear overnight.
I think actors and AEA should be aware of all newly discovered health risks. The risks should be discussed and evaluated as "on the job safety" issues and not dismissed. If the risk is serious and can be avoided, it should be.
This is work, not interpersonal choices in dating.
Pneumonia