Looking for a word with a more positive connotation than "infectious"

I recently was attempting to describe someone's smile. I wanted to describe it as being very 'infectious', or that it spreads very quickly and is contagious. However, as hard as I could try, I could only come up with words that have to do with infections or words that have a negative connotation.

How could I describe a person's smile if it has the ability to 'spread' to other people, as denoted above?

Thanks!

Solution 1:

Looking for a word with a more positive connotation than “infectious”

Their smile was,

irresistible adjective: too attractive and tempting to be resisted. "he found the delicious-looking cakes irresistible"

synonyms: enticing, tempting, alluring, inviting, seductive; Google irresistible

it was,

captivating adjective: captivating capable of attracting and holding interest; charming. "a captivating smile"; Google captivating

or,

engaging adjective: charming and attractive. "Sophie had a sunny personality that was very engaging"

synonyms: charming, appealing, attractive, pretty, delightful, lovely, pleasing; Google engaging

All these adjectives are so familiar as to be clichés, so a phrase,

Their smile was like a tune you couldn't stop humming.

Solution 2:

Infectious and contagious are very commonly used for this purpose, and no one really seems to object. If it really bothers you, you could say:

"He has a winning smile" - which means his smile 'wins people over', or, makes them feel happy too. (US)