function ydot=RHS_SEIR(t,y)


%  RHScovi
%
%    define the RHS of the model equations. 
%        variables are 
%  y(1): S, susceptible, uninfected individuals [number]
%  y(2): I, infected individuals [number]

%   array indexes of variables

nVar=4;iS=1;iE=2;iI=3;iR=4;

global PAR 

% most rates are 1/day

ydot=zeros(nVar,1);


% EQUATIONS 

% Time changes of susceptable individuals,

ydot(iS)= - PAR.Iinfect * y(iI) * y(iS);

% Time changes of Exposed animals 

ydot(iE)=  PAR.Iinfect * y(iI) * y(iS)-PAR.inc*y(iE);

% Time changes of Infected animals 

ydot(iI)=  PAR.inc*y(iE)- PAR.rec*y(iI)-PAR.Imort * y(iI);

% Time changes of Recovered animals 

ydot(iR)=  PAR.rec*y(iI);