Blossom end rot affects tomatoes, peppers, squash and other cucurbits.  The blossom end turns brown and the fruit rots.  The cause is a lack of calcium aggravated by an imbalance of soil moister and high salt levels.  Most garden soils have high levels of calcium from adding gypsum and the problem is likely caused by the inability of the plant to update nutrients due to root issues such as nematodes or root diseases.

Be sure your soil does not dry out nor become oversaturated with water.  It is not a caused by a pathogen nor can it be spread to other plants.  If the plant is not healthy, remove it and plant something different in the location.

http://ipm.ucanr.edu/PMG/GARDEN/VEGES/ENVIRON/blossomendrot.html