Well, you will need a collection (an array or a 2 dimensional array) that will contain a type of illnesses and a list of symptoms that belong to that illness. I will use a multi-dimension integer array to keep things simple. Here's a snippet:
// illness definition
int flu = 9
int cold = 8;
// symptom definition
int runningnose = 10;
int cough = 9;
int illnesses[5][10]
ilness at position 0 will have the following symptoms:
illness[0][0] = 10;
illness[0][1] = 9;
Where 10 and 9 is a place holder for a particular symptom (like running nose).
The first 0 in the first bracket denotes a particular illness (like the flu).
So, first you have to come up with a way to define your illnesses. In this case, I use numbers. This will be your program's dictionary.
Now, you will need to get the user's input and compare it to the dictionary that was previously defined. You will have to loop through the entire multi-dimensional array and see which illness match up best. I would save the illness that has the most match in a temporary variable. Finally, by the time the program exit the loop, you will have an illness that match best, unless there is a tie. In that case, you will have to return multiple illnesses if you so choose or you can simply return the first illness. Now you can format the result in whatever fashion you want because the program knows what illness it is. In my example, you could simply return 8 for a cold, but of course the number 8 to a human don't mean much.
Hope this gets you going.