the six classes of second-line drugs. This definition should no longer be used, but is included here because many older publications refer to it. The principles of treatment for MDR-TB and for XDR-TB are the same. The main difference is that XDR-TB is associated with a much higher mortality rate than MDR-TB, because of a reduced number of effective treatment options. The epidemiology of XDR-TB is currently not well studied, but it is believed that XDR-TB does not transmit easily in healthy populations, but is capable of causing epidemics in populations which are already stricken by HIV and therefore more susceptible to TB infection. Epidemiology of drug-resistant TB A 1997 survey of 35 countries found rates above 2% in about a third of the countries surveyed. The highest rates of drug-resistant TB were in the former USSR, the Baltic states, Argentina, India and China, and was associated with poor or failing national Tuberculosis Control programmes. Likewise, the appearance of high rates of MDR-TB in New York city the early 1990s was associated with the dismantling of public health programmes by the Reagan administration. Paul Farmer points out that the more expensive a treatment, the harder it is for poor countries to get. Farmer sees this as verging on denial of basic human rights. Africa is low in quality of treatment partly because many African cultures lack the 'concept of time' essential to the schedule of administration. MDR-TB can develop in the course of the treatment of fully sensitive TB and this is always the result of patients missing doses or failing to complete a course of treatment. Thankfully, MDR-TB strains appear to be less fit and less transmissible. It has been known of many years that INH-resistant TB is less virulent in guinea pigs, and the epidemiological evidence is that MDR strains of TB do not dominate naturally. A study in Los Angeles found that only 6% of cases of MDR-TB were clustered. This should not be a cause for complacency: it must be remembered that MDR-TB has a mortality rate comparable to lung cancer. It must also be remembered that people who have weakened immune systems (because of diseases such as HIV or because of drugs) are more susceptible to catching TB. Children represent a susceptible population with increasing rates of MDR and XDR-TB. Since diagnosis in pediatric patients is difficult, large number of cases are not properly reported.