Multipurpose spray and Lysol
These are essentials. If you do not have these two things already, go to the closest store and grab them. I barely used cleaners last semester and I was sick almost every other week. With such a small space and so many people to spread germs, it is essential that you disinfect as often as you can. I try to clean high-touch surfaces each day including the door knob, dresser handles, and even where you open the fridge. Living in a clean space will avoid the buildup of germs, so you should find you are feeling a lot better!
Open the windows!
Even if it is cold outside, just do it. I am not a fan of leaving the windows open for an extended amount of time because I am always cold, but it really helps. Such a stuffy small dorm room leaves air uncirculated so it can feel musty even if you keep it clean. Opening the windows will air out your space and fresh air will take out all of the re-used air that could be making you sick.
Dress warm - don't forget your coat
Even when it would get colder outside by the end of the first semester, I would always run out the door without a coat. When you need to be at class, it is easy to tell yourself that you will be able to brace through the colder temperatures for the few minutes it takes to walk to class - BUT this is a really bad habit. The cold makes can inflame your sinuses and throw off your temperature which can make you sick almost instantly. Wearing a coat (maybe gloves and a scarf, too) is an easy way to avoid getting sick.
Taking vitamin supplements & immune system support
Vitamins are so important! I take probiotics, vitamin D, and a multivitamin each day. Also, when I come back from going out or being in crowded areas for a long period of time, I try to take Emergen-C. It gives you a ton of vitamin C which is an immune system boost. It won't fix your sickness, but it will avoid it from starting... so I highly recommend it! It has been working for me so far, knock on wood.
Stop sharing so much!
Sharing food, clothes, school supplies, chairs, space in your room, etc. are NOT ideal when it is the season of being sick. Colds and other respiratory infections spread so easily so drinking the same drink as someone else or allowing them to sit right next to you in your dorm will increase your chances of catching something. Keeping your things to yourself (when you can), or cleaning your spaces if you must is the best option.
Remember to drink water.
Keeping hydrated is essential. I am terrible at remembering to drink water, but I have gotten better at it. I try to keep a water bottle with me at all times to motivate myself to stay hydrated. Being dehydrated during sickness season will not help you. Water has so many benefits that you might as well make it part of your routine. Even drinks like sparkling water can make it easier to drink, or different types of tea have benefits and will hydrate you as well.
Get enough sleep.
This is something I have found to be really important - that I definitely did not follow my first semester. Staying up until 2:00am was so normal on a school night - even if it was just watching Netflix. But, showering and being asleep earlier will give you the sleep you need. A more normalized schedule of going to bed at a decent time and waking up before noon will normalize your body. Saying no to going out 4 nights in a row is okay too... for the most part be sure that you are not sleep-deprived and this should help you out.