Blossom end rot is a physiological disorder—that is, a disfigurement of the fruit. It’s not “contagious,” as it won’t spread to the rest of the plant.
Use a balanced, all-purpose vegetable fertilizer or tomato fertilizer at planting time to ensure your plant gets what it needs, but don’t overdo it.
Tomatoes have long roots that benefit from long, consistent watering to make sure the moisture soaks in deep to where the mass of the roots are.
Try to avoid deep cultivation of the soil near the plant roots after fruit set. If you need to control weeds, hand pull them or scrape the soil lightly with a hoe to remove them.
If you find that season after season, your fruits are continually afflicted with blossom end rot despite your most valiant efforts to water them, it’s time to take a soil test.
Just be sure to remove any rotting fruit as soon as you see it—you want the plant’s energy to go into growing good, healthy tomatoes!