Definition of infectious:

• Having the quality of infecting or tainting; apt to spread; corrupting.
• Having the quality of infecting; contaminating; apt to spread.

Usage examples for infectious

1. " You'll have to do as I say, madam, or be turned out of the hotel," said he, with an infectious grin.  The Purple Parasol by George Barr McCutcheon

2. That old- fashioned feeling is infectious.  No Hero by E.W. Hornung

3. The warning was unnecessary, for the stillness of snow is infectious, while the beauty of the scene seemed to command silence.  The Sowers by Henry Seton Merriman

4. Selingman was the giver of the feast and his spirits were both wonderful and infectious.  A People's Man by E. Phillips Oppenheim