However, depending on what you're referring to, some options include:
For insects:
* Swarm - This is the most common term for a large group of flying insects, especially locusts, bees, or flies.
* Cloud - This is often used for a dense mass of insects, like gnats or mosquitoes.
* Host - This is more specific, referring to a large group of insects infesting a particular location or organism.
For diseases:
* Epidemic - This refers to a widespread outbreak of a disease in a particular area.
* Pandemic - This refers to a global epidemic, affecting a large population.
* Outbreak - This is a general term for a sudden increase in cases of a disease.
For other things:
* Scourge - This is a metaphorical term used for something causing widespread suffering or destruction.
* Infestation - This refers to a large and overwhelming number of unwanted things, like pests or parasites.
The best collective noun to use will depend on the specific context and what you are referring to.