However, depending on the context, you could use:
* A plague of diseases: This is a common and effective way to describe a large number of diseases, especially when they are widespread or harmful.
* A scourge of diseases: This implies a more serious and devastating impact.
* A constellation of diseases: This suggests a diverse collection of diseases, perhaps related in some way.
* A spectrum of diseases: This highlights the range and variation within the collection.
Ultimately, the best choice will depend on the specific situation and the effect you want to convey.