I think you should probably start with learning the main ones. Under the main soinc page for Disease Detectives, there is a link to the CDC page of Foodborne Illness. Under that link, it talks about main foodborne illnesses. Definitely know those, be familiar with them, know how they are transmitted, common symptoms, etc. Once you have that, then you can start looking at the other diseases under the link you have. Personally, for Invitationals, you should be able to get by just knowing the basic diseases. Even for State and Nationals, I highly doubt they would give some totally unknown disease- the scenario(s) should be on pretty familiar diseases. If they did give you a matching part or something on the test, it would be pretty common diseases, it wouldn't be something that you've never heard of.