The most common types of illnesses are caused by contaminated food and water. Mild cases of traveler's diarrhea may respond to Imodium (known generically as loperamide) or Pepto-Bismol. Be sure to drink plenty of fluids; if you can't keep fluids down, seek medical help immediately.
Infectious diseases can be airborne or passed via mosquitoes and ticks and through direct or indirect physical contact with animals or people. Some, including Norwalk-like viruses that affect your digestive tract, can be passed along through contaminated food. Speak with your physician and/or check the CDC or World Health Organization websites for health alerts, particularly if you're pregnant, traveling with children, or have a chronic illness.
Shots and Medications
Unless you're arriving from an area that has been infected with yellow fever, typhoid, or cholera, you don't need to get any shots or carry medical certificates to enter Australia.
Australia is relatively free from diseases prevalent in many countries. In the far north there have been occasional localized outbreaks of dengue and Ross River fever—just take the usual precautions against mosquito bites (cover up your arms and legs and use ample repellent), and you should be fine.
Familiar brands of nonprescription medications are available in pharmacies. Note that Tylenol is usually called paracetamol in Australia.
Specific Issues in Australia
Australian health care is excellent, with highly trained medical professionals and well-equipped hospitals. Hygiene standards are also high and well monitored, so you can drink tap water and eat fresh produce without worrying. You may take a four weeks' supply of prescribed medication into Australia (more with a doctor's certificate)—if you run out, pharmacies require a prescription from an Australian doctor. The quickest way to find one is to ask your hotel or look under "M" (for Medical Practitioner) in the Yellow Pages.
Sunburn and sunstroke are the greatest health hazards when visiting Australia. Remember that there's a big hole in the ozone layer over Australia, so even on cloudy days the rays of light coming through are harmful. Stay out of the sun in the middle of the day (the hottest hours are generally 11 am–2 pm) and, regardless of whether you normally burn, follow the locals' example and slather on the sunscreen. Protect your eyes with good-quality sunglasses, and try to cover up with a long-sleeve shirt, a hat, and pants or a beach wrap whenever possible. Keep in mind that you'll burn more easily at higher altitudes and in the water.
Dehydration is another concern, especially in the Outback. It's easy to avoid: carry plenty of water and drink it often.
No rural scene is complete without bush flies, a major annoyance. These tiny pests, found throughout Australia, are especially attracted to the eyes and mouth in search of the fluids that are secreted there. Some travelers resort to wearing a face net, which can be suspended from a hat with a drawstring device.
Some of the world's deadliest creatures call Australia home. The chances of running into one are low, particularly in urban areas, but wherever you go, pay close heed to any warnings given by hotel staff, tour operators, lifeguards, or locals in general. The Outback has snakes and spiders, while on the coast there's everything from sharks to octopi, stonefish, and jellyfish. In northern Australia, rivers, lakes, billabongs, and even flooded streams and creeks are home to estuarine crocodiles. The best advice is to always be cautious, and double-check the situation at each stop with the appropriate authority.
Australian coastal waters are also home to strong currents known as "rips." Pay close attention to the flags raised on beaches, and only swim in areas patrolled by lifeguards. If you get caught in a rip, the standard advice is never to swim against it, as you rapidly become exhausted. Instead, try to relax and float parallel to the shore: eventually the current will subside and you will be able to swim back to the shore, albeit farther down the coast.