In colder climates, houseflies survive only with humans. They have a tendency to aggregate and are difficult to dispose of. They are capable of carrying over 100 pathogens, such as those causing typhoid, cholera, salmonellosis,[9] bacillary dysentery,[10] tuberculosis, anthrax, ophthalmia, and parasitic worms.[11] Some strains have become immune to most common insecticides.[12][13]