The diseased white people found that their diseases were useful for killing off the clean native Americans.
The diseased white people introduced many diseases to America such as small pox:
In 1507 smallpox was introduced into the Caribbean island of Hispaniola and to the mainland in 1520, when Spanish settlers from Hispaniola arriving in Mexico brought smallpox with them. Smallpox devastated the native Amerindian population and was an important factor in the conquest of the Aztecs and the Incas by the Spaniards. Settlement of the east coast of North America in 1633 in Plymouth, Massachusetts was also accompanied by devastating outbreaks of smallpox among Native American populations, and subsequently among the native-born colonists. Some estimates indicate case fatality rates of 80–90% in Native American populations during smallpox epidemics
The British army at least once attempted to use smallpox as a weapon, when they gave contaminated blankets to the Lenape during Pontiac's War (1763–66). It is suspected, but not confirmed, that biological warfare was used against the Indians at other times as well.
Some historians have speculated the British deliberatlry introduced smallpox to the Aboriginal population of Australia. Some of the of the British soldiers sent with the First Fleet had served in Pontiac's War, in which smallpox was used as a weapon by British against native Americans.