"The protection of the environment, the health of human and animal populations, the productivity of farming and other commercial sectors, all these constitute good reasons for maintaining effective control over infectious diseases."

Vaccination is usually the preferred method of control, but for some infectious diseases with low environmental disease challenges (ie. in disease-free countries) or a long incubation period (ie. Tuberculosis), then culling (of animals) or medical treatment (ie. antibiotics, etc.) may become the optimal method of control [6]. This is particularly true where the relevant vaccines are less than 100 per cent efficient, or the excessive use of vaccines can create carrier individuals which subsequently allow the disease to persist and reappear [7].

It is however, the measured level of subclinical disease which can indicate whether carrier individuals are likely to occur - a high level of measured subclinical disease translates into a high likelihood of carriers. The solution to this problem lies in increasing the surveillance zones around foci (ie. areas) of subclinical disease, and to also minimize repeated or regular vaccination programmes. It is worth noting that whilst carriers remain a problem for disease-free regions, they are not an issue for endemic or semi-endemic areas (where disease is found frequently). Carriers will be almost irrelevant in regions of endemic disease since the disease challenge will already be high. Hence vaccination is generally, the most effective disease control tool for endemic regions [19]. Carriers will also be less relevant for diseases where vaccination remains the only means of disease control.

It is also important to remember that specific control methods have been employed within different regions because historically they work.