The weather is hot and visiting a local swimming pool seems like a good option, but Victorian health authorities are warning there's a nasty threat lurking beneath the surface.
As temperatures soared up to 46C in parts of the state on Tuesday, acting Chief Health Officer Brett Sutton issued a germ warning to swimmers.
"Even in the best maintained pools, germs on your body can wash off and contaminate the water," he said.
Cryptosporidium is one waterborne threat which can cause respiratory and gastrointestinal illness.
There were 780 cases of "crypto" reported to the health department last year and six of them were linked to aquatic facilities, Dr Sutton said.
They are most common in summer when more people use swimming pools, he added.
"You and your children are more likely to be infectious when not feeling well. Showering with soap before swimming will help keep germs out of the water."
Dr Sutton said cases tended to spike over summer, linked to children using swimming pools.
"It's not increasing, but this is definitely the season to look out for it," he told AAP.
"We've had good years and bad years over time. A couple of years ago, we did have 18 pool associated outbreaks."
The best prevention measure is to ensure children wash and clean themselves properly before entering the water.
They should also avoid swallowing water, as this is how the infection is caught.
If children have gastro or are recovering, they must wait 48 hours after the last episode of diarrhoea before using a pool.
Nappies must also only be changed in the change room.
Parts of northwestern Victoria are forecast to experience heatwave conditions with Mildura due to reach 46C on Tuesday and Wednesday, 45C on Thursday and 39C on Thursday.
Wangaratta is forecast to reach above 40C each day until the weekend.
A total fire ban is in place across Victoria on Tuesday.
In Melbourne, the outlook is milder, hovering around 30C for the rest of the week and aided by sea breezes, but it's still hot on court for players at the Australian Open.
A heat health alert was issued by Dr Sutton's department, warning that forecast temperatures might pose a threat to some people.
"If you're going to the Australian Open, or attending any outdoor events, plan ahead. If you must go out in the heat, wear a hat and sunscreen and always take a bottle of water with you," Emergency Management Commissioner Andrew Crisp said.