One of the women slaves called Naseema relates that when she went to see the baby Imam in his cradle she greeted him with salaam and was replied to. She then sneezed and the Imam said: "Yarhamukallah - it is a blessing from Allah and you have 3 days immunity from death".
When you sneeze you should say Alhamdulillah.
When you sneeze you should put your hands or a tissue in front of your mouth so that you do not pass your germs to anyone else and give them your cold.
When someone else sneezes you should say Yarhamukallah, because Islam teaches us to care for each other.