The best source of Vitamin D is the sun. In the northern hemisphere, the springtime sun is stronger than in in winter and not as harsh as summer, making it easier to be outside longer. Consider making extra time to be outside, with exposed skin; for bonus points, be moving- walking, gardening or playing. (meditation is good too). Since it is the end of winter, this is also a good time to check your Vitamin D level to see how low it gets. Remember that the older you are, the more fat you have, and the darker your skin, make your vitamin D synthesis less efficient. Checking your level helps you know how much supplementation you need.
Help your body take care of you by drinking enough water; if you have a persistent cough, an accessible, economical treatment is being well hydrated.
Think of the cilia in your respiratory tract as hairs that protect your lungs. They work best when well hydrated- your body uses the water you drink to hydrate essential parts, including the cilia.
There are other things that can help, but reaching for water first, before things get severe can be a good idea.
Our bodies have great abilities to adapt to too much or too little water; because of this, we can be complacent and not notice when improving the quality or having a glass of water instead of a different beverage can improve how we feel. Filtered water in a glass or stainless water bottle is healthier than a plastic bottle.
If you are feeling hungry, listless or tired, have a big glass of water and notice if you feel better, there's a good chance it will help.