Aliens have been invading the United States for centuries. A joke? Not at all. In fact, thousands of foreign plant and animal species have arrived on U.S. soil. Some are brought in intentionally by humans; others enter accidentally on boats or with cargo on airplanes.
The Invasive Species Advisory Committee (ISAC) defines an invasive species "as a non-native species whose introduction does or is likely to cause economic or environmental harm or harm to human, animal, or plant health." Most foreign species don't have natural enemies or predators in their "adopted" environments, which allow them to reproduce or spread quickly.
Viruses, like the West Nile Virus, can be transmitted by non-native species, such as the Asian tiger mosquito. West Nile Virus can cause severe respiratory infections in humans. Non-native poisonous plants can be full of nasty poisons. For instance, the sap of the tree-of-heaven or the Chinese sumac tree can cause inflammation in the human heart muscle.
Blights, or diseases, of non-native origin have wreaked havoc on animal species and woodlands, dramatically reducing the populations of both. Some species, such as the Eurasian wild boar, have caused a great deal of damage to native plants and crops.
Non-native species can impact property values. For instance, the Formosan subterranean termite can cause massive structural damage to buildings. Recreational activities, such as fishing, can also be disrupted when native species are reduced or eliminated when another species has invaded and prey upon the native inhabitants.