AIBU to be super upset MIL kissed baby with a cold sore?

[KLDL]

I went to my MIL’s house today with my 5-month-old. She has always been quite OTT with the baby and has crossed boundaries before – for example turning up at our house with a cold and still holding the baby when I said I wasn’t comfortable, and even when baby was only 6 weeks old she came over with obvious cold symptoms, wore a mask incorrectly and still insisted on holding her. When baby was born she also told the whole family before we were even out of hospital, despite my husband asking her not to.
Anyway, today we went over for lunch. As soon as I arrived she said she had a cold, and I could see she also had a scabbed cold sore. For context, cold sores are caused by the herpes simplex virus (HSV-1), which can be extremely dangerous for babies – in rare cases it can lead to serious infection affecting their skin, eyes, or even organs, and can be life-threatening.
I pointed this out before she held the baby and specifically said please don’t kiss her because of the cold sore. She seemed horrified and reassured me she wouldn’t.
An hour later she tickled the baby and then kissed her tummy. I wiped it when she wasn’t looking but didn’t say anything (which I now regret). Then about 10 minutes later she kissed her properly on the cheek. I immediately challenged her and she just looked confused and said “yes, why?” I reminded her again about the cold sore, took the baby back and wiped her face and skin.
On the way home I noticed baby had scratched her cheek and had a small cut, which has really worried me in case anything could get into it. I took her straight home and washed her.
MIL has early dementia / mild cognitive impairment. We would never leave the baby alone with her anyway, but she does get annoyed when I take the baby back to change or feed her as she feels it takes away from her time.
I feel really upset and honestly don’t want to be alone with her anymore as I don’t feel comfortable enforcing boundaries, especially when she either ignores them or forgets them. My husband is supportive, but we’re also aware that even if we explain things again, she may not remember due to her condition.
AIBU to feel this strongly, and how would you handle this going forward?

Hi guys, thanks so much for your honest feedback - i take the point that she has the mild cognative impairment and need to speak up more. Please dont judge as there are other things going on deeper than you can fit into a 5 paragraph summary that have made it extremely difficult to advocate for myself in the past - its not easy being a daughter-in-law!! But I definitely take the point that I need to step up now, set better boundaries and be more firm. The suggestions of how to deal with this in the moment are amazing, just being firm and changing the subject. I will be trying this next time.

Someone also commented and said I "obviously dislike" my mum in law - this is definitely NOT the case, I love her deeply but obviously I wrote this a few hours after it happened and was EXTREMELY upset and it may have seemed that way, but we actually have a good relationship most of the time, theres just been quite a few boundaries pushed since baby was born.

Agaun, thank you for some of the advice here, its so appreciated.

I would ring the gp/111 and see if anything can be prescribed to help. She could end up very seriously ill.

Essentially you have to accept MIL being upset. No holding the baby when she has a cold sore at all Or she is ill. Its sad, she has an illness that should be treated with compassion, not at the expense of your child's health.

You really shouldn't be expecting a person with dementia to manage this kind of thing. So YABU there.