Ecotourism now accounts for twenty per cent of tourists. It should provide a sustainable alternative to overuse of natural resources. However, tourists may introduce new diseases to animal populations. Mongooses and meerkats in Botswana have died from tuberculosis caught from humans, and gorillas in East Africa picked up new internal parasites after the introduction of tourism. Moreover, the presence of humans in increased numbers has been shown to stress polar bears, penguins, dolphins and rainforest birds, affecting their natural routines and reducing breeding success.