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.