All passengers who do not possess a valid pre-screening certificate for tuberculosis and are seeking leave to enter the UK for more than six months, and who have travelled from a country where the tuberculosis incidence rate is higher than 40 cases per 100,000 of the population, are referred to the Port Medical Inspector for tuberculosis screening. Around 135 countries currently have this incidence rate of tuberculosis and can be found at the World Health Organisation website:
http://www.who.int/en/