The most common illnesses for travellers to get are the common cold and the flu. This is because you're probably spending a significant amount of time in an enclosed space with a whole lot more people than you're used to being around.
While planes, trains and other modes of mass transport do their best to keep the space clean and the air appropriately filtered (with extra measures having been added following the pandemic) you're still more likely to get sick if anyone near you has a viral infection.
To help avoid this, wash your hands regularly and sanitise them if hand washing facilities aren't available. Checking travel advice for any illnesses that are common in the area you're looking to visit and taking preventative measures such as vaccination if available is wise. This should be in addition to your regular health regime and travel vaccines.
You may also wish to speak to your doctor about taking a vitamin or supplement to help boost your immune system before and during your travels.