When people say “async is viral” in Rust, they mean that once you make one function async, that change tends to ripple through the rest of your code. Any function that calls it usually has to become async as well so it can await the result. In turn, the callers of those functions often need to become async too.

This propagation can continue all the way up the call stack until you reach your application’s entry point. The main exception is when you introduce an explicit synchronous-to-asynchronous boundary, such as by using block_on, which drives the future to completion without requiring the caller itself to be async.