Camelia with yellow leaves

I presume that this is planted in the ground and not in a pot? Camelias are acid lovers.
The soil is too alkaline for it. Obviously it has survived to become a large plant. If your soil is naturally alkaline it may have originally been planted in a lot of acid compost but the roots are now growing out from this.
Try watering it with chelated iron. This is sold as a soil acidifier to make hydrangers red. The brand I have used is "Miracid" soil acidifier and plant food.
I imagine it is too big to pot but planting it it in ericaceous compost is the other solution.

I forgot to say that Strpy cat.