The following definitions are taken from the Belgian Regional Decrees regulating contained uses of GMOs and pathogens.

- Class of risk 1 : micro-organisms known as non pathogenic for the man, the animal, the plant and not-harmul for the environment or presenting a negligible risk for the man and the environment at the laboratory scale. This class includes, beside organisms whose harmlessness was proven, strains which can be allergens and opportunistic pathogens.

Human pathogens

- Class of risk 2 : micro-organisms that can cause human disease and might be a hazard for directly exposed persons; they are unlikely to spread to the community. There is usually effective prophylaxis or treatment available.
- Class of risk 3 : micro-organisms that can cause severe human disease and present a serious hazard for directly exposed persons. They may present a risk of spreading to the community. There is usually effective prophylaxis or treatment available.
- Class of risk 4 : micro-organisms that cause severe human disease and are a serious hazard for directly exposed persons. They may present a high risk of spreading to the community. There is usually no effective prophylaxis or treatment available.

Animal pathogens

- Class of risk 2 : micro-organisms that can cause disease in animals and present, at different levels, one or other of the following characteristics: limited geographical importance, no or weak interspecific transmission, no vectors or carriers. The economic and or veterinary significance is limited. There is usually effective prophylaxis or treatment available.
- Class of risk 3 : micro-organisms that can cause serious disease or epizootics in animals. Interspecific diffusion can be important. Some of these pathogenic agents require the installation of sanitary regulations for species indexed by the authorities of each country concerned. Medical and/or sanitary prophylactic measures are available.
- Class of risk 4 : micro-organisms that cause extremely serious panzotics or epizootics in animals with a very high mortality rate or dramatic economic consequences in the affected farming-regions. Either no medical prophylaxis is available or only one exclusive sanitary prophylaxis is possible or obligatory.

Plant pathogens

- Class of risk 2 : micro-organisms that can cause plant disease, but that does not present an increased risk of epidemic in the event of accidental dissemination in the Belgian environment. They are ubiquist pathogens for whom prophylactic and therapeutic means exist. Nonindigenous or exotic phytopathogen micro-organisms which can not survive in the Belgian environment because of absence of hosts or plant-targets, or favorable climatic conditions also belong to the class of risk 2.
- Class of risk 3 : micro-organisms that can cause in the plant a disease of economic or environmental importance for which treatments are non-existent, difficult to apply, or expensive. The accidental dissemination of these micro-organisms can increase the risks of local epidemics. Exotic stocks of micro-organisms usually present in the Belgian environment and not listed as quarantaine micro-organisms also belong to this class of risk.