To stop being Sick in Project Zomboid, you will need to head to a safe location such as a home and rest for a few days. It will take time to get rid of Sick moodles, and the best way to do it is by resting.
We also recommend making sure you eat well until you get the green well-fed moodle. Keep eating until you get the well-fed moodle for a few days, and it will help you heal and get better faster.
On top of being both warm, rested, and well-fed, not being tired and getting plenty of sleep will further add to your recovery from being sick.
So again, hold up at either your base or a safe location and give yourself time to heal. It will also help if you are stocked up with canned food and water, so you can easily last a few days without going out on loot runs.
But, if your sickness has not gone away after a few days and has increased to Nauseous or, worse, Fever, you are most likely infected. If this is the case, there is nothing you can do as currently there is no cure for the Zombie Virus.
For more guides on Project Zomboid, we at Pro Game Guides have you covered with an ever-expanding roster of guides! But If you need help with another moodle like depression, check out All Project Zomboid Moodles and how to deal with them