First, diseases affect cells by damaging, killing, or infecting cells.
Second, diseases affect tissues by making it sometimes thinner or weaker.
Third, diseases affect the organs by making it function improperly.
Fourth, diseases affect the organ systems by making it function improperly too, because organs in the particular system is affected, organ system will work improperly, as I said.
Fifth, organism with the disease will take complications, he/she may become weak, bedridden, or sick, the worst case is to die. :(
-- Take care of yourself, 1 affects many.