Hello Mailme, if it is Blossom end rot (BER) it is not a disease but a calcium deficiency made worse by irregular watering. It will not spread from one plant to another.
The bigger tomatoes are more susceptible - Marmande and the like. I got a nil crop from them and don't grow them anymore.
The smaller tomato varieties don't suffer from it - so I stick to them.
If you get rid of the wasted fruits the ones to come may be fine. Good luck.

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