The phrase "infectious smile" is correct and usable in written English. You can use it to describe a person's smile which is happy, cheerful, and so expressive that it propels others to smile too. For example, "He always had an infectious smile that could lighten up the mood of any room."
Who could forget his infectious smile?
Chick Magnet with an infectious smile.
She had an easy laugh and a wonderfully infectious smile.
islandchick Nicola Adams has such an infectious smile.
We'll miss his infectious smile and engaging personality.
Remembered for his infectious smile, dapper attire, and magnetic personality.
We will forever miss his bear hugs and infectious smile.
With his infectious smile, Mr. Maadi dismissed the worries.
We will all miss his wonderfully infectious smile.
He does, however, recall her "infectious smile" and "incredibly addicting" laughter.
She has the timeless elegance of Billie Holiday and the infectious smile of Josephine Baker.
Ludwig does not simply clarify my doubts with English writing, it enlightens my writing with new possibilities
Simone Ivan Conte
Software Engineer at Adobe, UK