In the 2010 movie "Hot Tub Time Machine," four friends in their 30s — played by John Cusack, Craig Robinson, Rob Corddry and Clark Duke — are transported back to 1986 after jumping into a hot tub that turns out to be a time machine.

Luckily (spoiler alert!), when they eventually take the hot tub back to the future, they discover that their lives have been changed for the better by revisiting the past.

Clearly, hot tubs can be a great experience. But don't let your steaming, bubbling, tight-muscle relaxer turn into a Petri dish.

High temperatures make it hard to keep disinfectants at adequate levels, and stubborn bacteria can deliver something just as unexpected as a trip back to high school.

Pseudomonas aeruginosa can get under your suit (if you're wearing one) and infect hair follicles, causing a hot-tub rash.

And Legionella can cause a potentially life-threatening type of pneumonia called Legionnaire's disease if you breathe those microbes in from the mist coming off the tub.

Here's how you stay safe:

• Get pool test strips. Test water for free chlorine, which should be at 2-4 parts per million (ppm), or bromine (4-6 ppm). The pH should fall between 7.2 and 7.8. These levels cut down on the chance that bacteria might call your hot tub home.

• Make sure hot-tub walls are smooth, not sticky or slimy.

• Make sure the sanitizing pump is humming.

• Shower before you dip; skin creams and other products can absorb disinfectants from the water, leaving less to kill bacteria.