Listen, there's no avoiding the fact that Dragon Con is a major vector for illnesses. The CDC (based in Atlanta) identified D*C2009 as a potential source of the spread of Swine Flu. That turned out not to be an issue that year (the Penny Arcade Expo did result in numerous cases, however), but that doesn't mean that in future years you're not going to be shaking hands with a carrier of some new superflu for the media to panic about. Basic health care prevention steps will go a very long way towards A) keeping you healthy, and B) preventing CNN and Fox News from decrying Dragon Con as the worst thing since the holocaust.

Media aside, being in a closed environment with recirculated air being breathed by you and 70,000 (Dragon Con made a new attendance record in 2015, yay) other people -- it pretty much turns the hotel and AmericasMart into a petri dish.

To that end, start calling around about Flu Shots early to mid-August. This cannot be stressed enough.  Many places do get them that early and it takes TWO WEEKS for the vaccine to reach proper protective potency.    Check with your local grocery stores, your job (many offer flu shots for their employees), and all the pharmacies in your area (who usually will let your insurance cover it, and if not, will only charge a little for the shot).  

Stay Healthy

The last thing you want to deal with when you get home is con crud. To avoid, consider the following:

• Take vitamins leading up to and during con. Ensure that your immune system is as strong as possible. If you decide to take Airborne (or some generic equivalent), don't take more than 2 or 3 per day as the high doses of vitamins can be damaging to your health.
• Wash your hands frequently. Hand sanitizer isn't a good substitute for soap and hot water, as it just pushes dirt and germs around rather than eliminate it, but is better than nothing.
• Don't share drinks. Try to keep your own, or only share with those you know. This is for safety as well as sanitary reasons.
• Get enough sleep. Try to get at least five hours a night if possible.
• Get enough food. Try to get at least one decent (wholesome) meal a day and remember to drink some juice and water to counterbalance the amount of caffeine and alcohol you're likely to be consuming. Protein and granola bars sound like a good idea until you've eaten four in one day -- pick up a fruit cup or something.
• Practice Safe Sex At Dragon*Con.

Hygiene

While it can be easy, and even beneficial, to break out of your daily routine for the sake of Dragon Con, hygiene habits are cast aside far too often. As the convention goes on, it's important to remember one thing above all others: WE CAN SMELL YOU. And it's generally not pleasant.  It is also somewhat unavoidable due to crowding. Even people who showered before leaving their room start sweating in the Atlanta heat within moments of stepping outside, or  being in a costume for more than a few minutes.

Body odors aside, it is critical that all attendees realize that not only does regular bathing make you a more pleasant person to be around, it's critical for health reasons. With the ongoing spread of whichever Flu is going around, the CDC has identified conventions specifically as being a critical opportunity for the spread of the disease. Cleaning yourself regularly will go a very long ways to avoiding catching illnesses, but it will go even further in helping prevent you, yourself, from becoming a modern day Typhoid-Nerdy. As such, the following is a list of regular hygiene activities to help keep you and everyone else healthy, and to assist you in keeping your foul vapors to your damn self.

Minimum Hygiene Requirements

• Bathe daily. DAILY. With soap, and hot water. You read this now and smirk because you think it's a joke. It is not. - MANDATORY
• Wash your hands after using the restroom. - MANDATORY
• Wash your hands before every meal. - MANDATORY
• Do not stick your fingers in your mouth in public: that's nasty. - MANDATORY
• Use deodorant. Ok, that's not so much for your health, but at least it will make you more presentable around others.
• Keep stick deodorant on you in case you didn't do an adequate job of deodorizing.  Spray deodorant causes fumes in the air. 
• Perfume/cologne/AXE Body Spray is not a replacement for this.
• Brush your teeth at least once daily. - MANDATORY
• If you plan on wearing a single costume for more than 6 hours, air it out and/or use Febreze. Its hot, crowded, and you will sweat.
• Incontinence pads are great for catching sweat and keeping it off your skin. If you use them or incorporate them into your costume, CHANGE THEM DAILY. 
• There are also microclimate cooling vests, but they are very expensive so unless you're a hardcore cosplayer or prone to heatstroke, they're not an easy counter-sweat method. 
• Change underwear daily. - MANDATORY

Recommended Hygiene Habits

• Brush your teeth after every meal
• Use floss and mouthwash to go along with this.
• Breath mints are a good idea for when the above isn't an immediate option.
• Keep a small bottle of hand sanitizer on you at all times.
• Rub it all over your face. Aww, yeah, you like that don't you.
• If you're sensitive to harsh hand sanitizer, undiluted tea tree oil also kills germs (but it STINGS). 
• Change your clothes at least once daily.  If your shirt is wet to the touch, you need to go change it (or buy a new one in the dealers room!)
• Please remember to use perfume/cologne very sparingly. In tight confines, and mingling with the odors of those who chose not to follow this guide, it can cause temporary feelings of illness and/or ill-will towards you, specifically.

Further Reading 

3-2-1 Rule

Con Etiquette

Attire

Con Crud

Con Funk

Advanced Nutritions and Nommings