I know I may be restating some of the comments made by previous contributors of this thread, but Disease Detectives is a very simple and fun event. The CDC resources, tests, and samples are extremely good resources that you should really embrace. You know you've fully used this resource only when you feel like you've done every practice test at least 3 times.
Also, the wiki available on this website is pretty much an "all you need to know" guide to Disease. Just make sure you thoroughly have memorized and understood this material. You'll be fine. Also, try to use the CDC glossary and get familiar with as many words and definitions as you can. If you speak in epidemiology's language on the test, you'll do very well. When answering questions, sound like you know what you're talking about (hopefully you do); use words like "susceptible" instead of "more prone," use words like "dermal" instead of "skin contact." It really helps you distinguish your team's knowledge from others. Often times, tests do
ask you to recite definitions. If you've studied the glossary, you'll get those defining questions. Usually, this is what separates first place from second/third place.
I'm not saying you should memorize the entire glossary. In fact, this probably won't really help as much. The goal is to really understand the information and work out problems as asked, not simply recite memorized facts. Speak the language that epidemiologists speak, and you'll do fine. Good luck!