  Spread of computer viruses can be modeled as the SIS
(susceptible-infected-susceptible) epidemic propagation. We show that in order
to ensure the random immunization or the targeted immunization effectively
prevent computer viruses propagation on homogeneous networks, we should install
antivirus programs in every computer node and frequently update those programs.
This may produce large work and cost to install and update antivirus programs.
Then we propose a new policy called "network monitors" to tackle this problem.
In this policy, we only install and update antivirus programs for small number
of computer nodes, namely the "network monitors". Further, the "network
monitors" can monitor their neighboring nodes' behavior. This mechanism incur
relative small cost to install and update antivirus programs.We also indicate
that the policy of the "network monitors" is efficient to protect the network's
safety. Numerical simulations confirm our analysis.
