Unwashed bottles can accumulate bacteria and can be dirtier than dog bowls and kitchen sinks, for example. Some of the bacteria can be harmful, and as a result illness and infections can occur.

"Several types of bacteria can be found on water bottles; essentially, they're the same ones you'll find on your hands or in your mouth," Dr. Bruni Nazario, WebMD medical director, said.

If you’re the only one reusing the container, it will still collect germs from your mouth and hands. Nazario said that the human mouth contains thousands of bacteria, and saliva is a natural breeding ground for germs.

"Normally, your immune system will play a role in attacking and destroying germs, but If you do develop an illness, it is typically diarrhea," Nazario said.

(FangXiaNuo/iStockphoto/Getty Images)

According to Dr. Josie Znidarsic, family medicine physician at Cleveland Clinic Wellness Institute, pneumonia, staph or strep infections are possible after drinking from a bacteria-ridden bottle.

Bacteria make their way on to your reusable bottle in multiple ways, such as dirty hands touching the screwcap, your mouth on the opening or straw of a water bottle or through the backwash of saliva as you drink.

TreadmillReviews.net tested for four types of bacteria on different types of water bottles. They tested for the following bacteria: gram-negative rods, gram-positive cocci, bacillus, and gram-positive rods.

"Slide-top bottles were found to have the highest amount of Colony Forming Units (CFU). This might be due to the lids that shut and can have more nooks and crannies for bacteria to lurk," Carly Johnson, a senior creative strategist with TreadmillReviews.net, said.

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Chuck Gerba, professor of microbiology and environmental sciences at the University of Arizona, has seen a bacterial problem with sports bottles that require the use of a finger to open and close the bottle.

"They tend to get fecal bacteria over time because you contaminate the push button with your fingers. So you could pick up a virus or bacteria that could make you ill because you may get it in your mouth the next time you use it," Gerba said.

According to Gerba, it may be best to stay away from these kinds of bottles.

(Image/Treadmill Review)

According to the Treadmill Reviews study, the best water bottle to drink from is one with a straw-top design.

"Straw-top water bottles only had 25.4 CFU on average, and only 8 percent positive cocci [bacteria]. In terms of germ harvesting, our study found this type to be the best to buy in order to avoid any germs," Johnson said.

When it comes to water bottles and bacteria, stainless steel is a better choice than plastic. Additionally, water bottles without crevices and tough-to-clean spots are less likely to host germs.

"Depending on the time between bottle cleanings you could see the growth of bacteria, fungi, or yeast. Depending on the type of bacteria, and the dose you ingest, you could get sick," Nazario said.

To properly wash and sanitize water bottles, a solution of one tablespoon of bleach per quart of water kills the bacteria. Wash reusable water bottles every day and don’t allow the water to sit all day before drinking it.

"Run it through the dishwasher or hand wash it thoroughly after every use. You can even use a weak bleach solution to sanitize your water bottle," Johnson said.

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For more safety and preparedness tips, visit AccuWeather.com/Ready.