Well a diary is a day to day record, so just start with a particular day and describe how things get worse and worse....how you first found out your relative has the disease, how you felt...and go on from there...use your imagination....you may have been moved to a different house, you may have been split up from the rest of your family, you may get snippets of information good and bad as to who else has been affected....and of course the ending can be either good or bad....
such as day 10..felt sick, noticed lumps on my hands
or day 10...my room has been unlocked, thank god we are safe....get the idea?