Mosquitoes can carry some diseases and give them to people or spread them between animals and people.
Tropical climates like Asia, Africa, Central America and the Pacific are higher risk.
Participating in certain activities can also increase your risk. For example, if you are outside a lot or near water.
If you're visiting countries where yellow fever is present, you'll have to show a valid yellow fever vaccination certificate to re-enter Australia. Your airline may also ask for a certificate before letting you board.
Your best protection is to avoid being bitten. Thre are simple steps you can take to prevent bites. You can also get vaccinated against some mosquito-borne illnesses.