I laughed because the other person had an infectious smile.

"Infections" refers to diseases that are contagious and can be passed on to others, like the Coronavirus. You definitely don't want that! InfectiOUS can also refer to diseases that are contagious. However, it can also describe a person who rapidly influences people in a positive way. Example: "Her enthusiasm was infectious." If you change ""infections" to "infectious", your usage in your example will be correct. However, it sounds a little bit unnatural. I would say, "I laughed because her smile was infectious."

You'd normally say - "His laugh was infectious" if you want to say that the other person made you laugh because of their laugh. "I when I saw how he smiled - I couldn't help but laugh" also okay. けどちょっと違い意味です