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State variables are: 
, 
, 
and 
, where 
represents the proportion of hosts (foxes) which are not infected and not immune at age 
, the other state variables are similarly defined. Parameter 
denotes the infection pressure (force of infection), measured in insults (exposures) per year; 
is the probability of immunity on exposure; 
is the rate of loss of host immunity; 
is the parasite death rate.
