Zinnia elegans, the most common and colorful group of plants. They are easy to germinate, easy to grow, and easy to love. They have been a main stay in gardens for years. I love to use them as colorful companions to a winding path in my garden for a colorful site. Zinnias, some reaching 3' tall, make great cut flowers that last.
Foliar diseases are the biggest problems with all types of common zinnia, and if you see enough plants shriveling up or powdery mildew recipe or dust with fine dusting sulfur. If leaves are curling up from the sides, which mildew causes, make sure you have space around the plant and try not to water the whole plant only the root line.
If you are fortunate enough to be able to grow picture-perfect zinnias all season, go for it. If not, all is not lost. Some people resow seeds at the base of existing plants in June or July and lift the old plants as they tire, allowing the young plants to mature in late summer and fall. I find that I sow the seeds in early summer after a lot of the spring perennials are done for the season.
If you have consistent problems, look for other popular cultivars derived from more disease-resistant species.
All zinnias benefit from deadheading. Proper cultivar selection and ample spacing between plants reduce disease problems. Full sun and lots of it.