kiarocks, instead of 'get(person.sex);' you should have 'person.getSex();'.
Also, instead of 'List person = ...' , try 'List<Person>persons = ...'
With '<Person>' in the expression, greenfoot will return the person, instead of just a reference to it.
As a reference, you cannot get 'sex' or 'infectionStage' or any other information;
but as a Person, you have access to all the information.
But, if you notice, there is a lag between the time a male and a female meet, and the time the birth occurs.
I just need to fiddle with it a bit to get to a point where SOMETIMES they all die off AND SOMETIMES they over-populate the world. (Not as easy as it sounds; especially if were trying to keep what is happening to represent what happens in real life).