Isolation is often impossible or impractical in a home garden. Some seeds may transmit certain diseases. A disease that infected a crop at the end of the growing season may do little damage to that crop. However, if the seed is saved and planted the following year, the disease may severely injure or even kill the young plants. Standard or heirloom varieties that are not cross-pollinated by nearby plants are good candidates.
Many gardeners successfully keep beans, tomatoes, lettuce, and peppers.
Plants you know are heirloom varieties are easy to save. Ask the person or organization you obtained the seed from how they did it. Some people like to experiment, but make sure you don't bet the whole garden on saved seed. When saving seed, always harvest from the best. Choose disease-free plants with qualities you desire.