It is possible that your tomato plants are suffering from blight. This is very infectious and can spread rapidly moving from bottom leaves up to stems and then the fruits themselves. If the stems are not yet affected, remove the infected leaves and dispose of them. Do not compost them as this will keep the disease for another year, I burn mine as the alternative would mean adding to the landfill problem. Check the stems of the plants to see if there are infected areas, remove all infected plants and dispose. If the stems are infected you may need to harvest all the unaffected fruit and dispose of all infected material. The harvested fruits will still ripen, There is no accepted organic control but copper sulphide sprayed on the leaves of tomatoes and potatoes can slow the progress though not elliminate the risk. However you would have needed to start spraying before the blight appeared as it is a sort of prevention. As I garden organically, I grow most of my tomatoes under cover, keep them well spaced and ventilated and avoid water on the leaves, so watering directly at the base. I also keep an eye out for both blight and for septoria leaf spot, the other infectious disease that can afflict solanaceous plants and particularly tomatoes. With my outdoor grown tomatoes I try to remove infected material as soon as I spot them,which means daily examination of plants.
Unfortunately if the fruits are affected they will rot and will infect any healthy fruits they are stored with.