Foot-and-mouth disease (FMD) is a highly transmissible and infectious viral disease inflicting cloven-hoofed animals. Cattle, sheep, pigs and goats are particularly at risk but FMD may also infect some wild species, including rats and deer. The mortality is highest in young animals. Prior to and during the occurrence of symptoms, the virus can be shed through exhaled air, lesions, milk, semen, and blood, making its transmission difficult to control. Direct contact between animals can transmit the disease, as can most animal products. The virus also has a remarkable capacity for remaining viable in carcasses, animal byproducts, water, straw and bedding, and even in pastures.

To date, seven immunologically distinct serotypes of FMD have been identified (A, O, C, SAT1, SAT2, SAT3, and Asia 1). There is no cure for the disease, but it can be prevented by vaccination. However, immunity to one serotype does not protect an animal against the other serotypes of FMD. Vaccines are available, but they must match the serotype and subtype of the virus present in the area. Also, vaccinated animals can become carriers without showing signs of infection. The disease has occurred in most countries of the world at some point in the last century. Only a handful of countries have never had FMD. Two major approaches can be considered to control the spread of FMD; (1) to cull all infected animals and those that might be infected ("stamping out"); or (2) to create an immunological barrier around the source of infection by vaccinating susceptible animals, in order to limit new infections and further transmission ("ring vaccination").

In Hong Kong, FMD is not as prevalent as in other countries, such as the UK. In order to prevent importation of FMD to Hong Kong, the Agriculture, Fisheries and Conservation Department (AFCD), prohibits the import of cloven-hoofed animals from the United Kingdom, France, Netherlands, Japan, South Korea, Taiwan, and all South East Asian countries with FMD. Nonetheless, new cases of FMD occur on local pig farms every year. From 1998 to 2002, there were about 3,000 cases of animals infected with FMD in Hong Kong. Half of the cases occurred in unvaccinated animals.