Manipulate[area=Round[NIntegrate[-c x (x-21) (x+1),{x,0,a}]];Plot[{-c x (x-21) (x+1)},{x,.001,a},PlotRange->{{0,21},{0,1500}},Frame->True,FrameLabel->{"Days","Number cells/mL"},LabelStyle->Directive[FontSize->14],PlotStyle->{Black},Filling->Axis,FillingStyle->If[area<7848,LightBlue,Red],PlotStyle->Black,Epilog->{Text[Style["Amount of infection = ",Black,FontSize->12],{5,1400}],Text[Style[area,Black,FontSize->12],{10,1400}],{Gray,Line[{{a,0},{a,-c a (a-21) (a+1)}}]}}],{{a,5, "Days"},0,21,.1, Appearance-> "Labeled"},{{c,1,"Immunity, c"},{1,0.9,0.85,0.8,0.6,0.4}}]