  In this paper, we introduce a nonresident computer virus model and prove the
existence of at least one positive periodic solution. The proposed model is
based on a biological approach and is obtained by considering that all rates
(rates that the computers are disconnected from the Internet, the rate that the
computers are cured, etc) are time dependent real functions. Assuming that the
initial condition is a positive vector and the coefficients are positive
$\omega-$periodic and applying the topological degree arguments we deduce that
generalized nonresident computer virus model has at least one positive
$\omega-$periodic solution. The proof consists of two big parts. First, an
appropriate change of variable which conserves the periodicity property and
implies the positive behavior. Second, a reformulation of transformed system as
an operator equation which is analyzed by applying the continuation theorem of
the coincidence degree theory.
