US Citizens travelling to Australia for tourism, or to visit friends and family, can apply for a Visitor/Tourist ETA visa which is valid for 12 months for stays of up to 3 months. It can be used for single or multiple entry travel, but employment is prohibited.
An ETA is an electronic visa, that can be purchased online at Australia ETA. There is a service fee of $20AU using American Express, Visa, Master Card, or Diners Club.
The ETA is filed electronically and there is no stamp or label in your passport. Most applications are approved in less than one minute.
No specific immunizations are required for travelers entering Australia from the United States, but always consult your physician or health practitioner about recommended procedures or immunizations, before traveling to foreign countries.