Nesting blue tit

Hi All, 

I have a blue tit thats nesting in my garden camera nest box and she looks nearly ready to lay eggs. I’m just a little worried as she seems to be scratching a lot and I think I saw something crawling on her so I imagine she’s got fleas. How bad will these likely be for any babies she has. It’s a brand new box not used before so she likely had them before she started building but I guess there isn’t much I can do given I can’t disturb the nest.

Photo just so you can see her.