First, the bad news: Our homes are filled with germs. A lot of germs. And while a safe level of germs can actually strengthen our immune systems, we certainly don't want them multiplying and leading to a host of health issues.

Now for the good news: While you may be surprised by this list of worst offenders, we've got the dirt on how to get them clean.

1. Kitchen sponge
It's used for everything from scrubbing pots to cleaning countertops, but according to a study by NSF International, a non-profit organization dedicated to public health and safety, that kitchen sponge is the nastiest thing in the house.

"Sponges are the absolute worst," says Kim Dunn, the owner and operator of Molly Maid in Aurora, Oak Ridges, King City and Nobleton, Ontario. "The bacteria sits overnight and all it is doing is multiplying in a moist environment."

The NSF International study found that, after three weeks, 70 per cent of sponges develop coliform bacteria, such as E. coli and salmonella, which can cause serious infection in children, the elderly, pregnant women and immune-suppressed individuals.

To clean: Dunn suggests filling the kitchen sink with just enough water to cover the sponge, adding about an eighth of a cup of bleach and leaving it overnight.

2. Kitchen sink   
A kitchen sink has 100,000 times more germs than a toilet, according to the Hygiene Council, a group of the world's top experts in hygiene and hygiene-related fields. Plus, the NSF International study discovered that coliform was found in 45 per cent of sinks.

Along with being regularly wiped down, sinks (as well as countertops and cutting boards) should be regularly disinfected; otherwise, bacteria from unwashed foods and hands are just being moved around rather than removed. Don't forget to disinfect the drain plug, too, especially underneath it, which is a breeding ground for bacteria and mould, cautions Dunn.

To clean: Wash the sink with hot, soapy water and scrub down its surfaces each night with a 25 per cent bleach/water solution, says Dunn. 
3. Bathroom walls
You're adamant about cleaning the toilet seat, rim and bowl, but have you taken a look at the surrounding bathroom walls? If you have a young boy in the family, well, aim is still likely a work in progress. Plus, when flushed, toilets spatter bacteria- and virus-contaminated droplets into the air, which land everywhere, including on your toothbrush and toothbrush holder, the latter of which was found by NSF International has the third most germs in the house.

To clean: NSF International recommends tossing the toothbrush holder in the dishwasher once or twice a week. And Dunn suggests using a mild detergent to wipe down the bathroom walls. You can also use bleach (25 per cent concentration in hot water), but be sure to spot test the walls first.

In the evening, swipe a bleach wipe or soft cloth with alcohol across the sink, taps and counters to disinfect them until cleaning day. And remember to always flush with the lid closed.

4. Keyboards and remote controls
"Keyboards are gross," states Dunn. "Our hands are always on them and we tend to eat when we're on the computer. So not only are you contending with the oils and bacteria from your hands, you have muffin crumbs and a splash of coffee."

A 2008 study by British tech magazine Which? Computing found that of the 33 keyboards examined by a microbiologist, four were potential health hazards and one had five times more bacteria than a toilet seat. If gadgets, including remote controls, telephones and cellphones, aren't cleaned regularly, transmission of colds and flus is child's play.

To clean: Once a month, turn your keyboard upside down and shake everything out of it, says Dunn. Use a makeup pad moistened with alcohol or a mild cleaner (do not spray it onto the keyboard) to wipe keys and crevices, as well as other household gadgets. And wash your hands regularly.

5. The shower
Talk about a wake-up call: A 2009 study from the University of Colorado found that showerheads harbour a number of bacteria, including Mycobacterium avium, that ride into the air when the shower is on. Although harmless for most people, M. avium could cause pulmonary disease in those with weak immune systems.

Meanwhile, the bacterium staphylococci lives in 26 per cent of bathtubs and showers, according to NSF International, and can lead to serious skin infections. Not to mention that slime at the bottom of your shower liner, which is a combination of mould, which can trigger respiratory problems, and tricophyton or fungus, which can cause athlete's foot and ringworm, says the Alliance for Consumer Education, an American non-profit organization dedicated to community health and well-being.

To clean: Soak the showerhead in vinegar overnight and scrub it with an old toothbrush, recommends Dunn. Squeegee tiles and glass doors after every shower, and clean the shower and tub weekly with a non-abrasive cream cleanser. Wash the liner every one to two weeks with bleach, and pull it out of the tub after showering and keep the bathroom fan on.