Bleach is our friend. It is the most effective, cheapest, most environmentally-friendly option for killing most of the terrible germs out there. And everyone seems to use it incorrectly. Too many people treat bleach more like a magic talisman than a disinfectant, filling a spray bottle and spritzing it on everything in the style of Windex Mom in My Big Fat Greek Wedding.
Bleach is like a comic book hero who has super strength only under the right circumstances. If the "bad guys" are hanging out somewhere wide open and clean, bleach is unstoppable. Other disinfectants like QAC's are more like stealthy assassins, sneaking in through the air ducts to get to the center of the bad guys' hideouts. The bad guys can learn to stop the assassins by simply blocking the air ducts, but they have no defense against the brute-force of bleach. (This is also why germs are not likely to develop resistance to chlorine bleach the way they can to the QAC's).
Except... like all good super heroes, bleach has a major weakness: dirt (all organic material, actually). The moment bleach touches dirt, it loses all its power. The bad guys can hide inside a microscopic ball of organic matter all day long. It's like a giant, multi-room mansion to them, and all bleach can do is peek in the windows.
Chlorine bleach does not work on:
Wood
Carpet
Plush toys
Most furniture
Fabric* (gray area, see below)
Dirty dishes
Dirty concrete
Dirty anything
Dirt
Let me repeat this, because I've talked to a lot of people about this, from all walks of life, and it's always a hard sell: I swear to you, no matter what you've heard (or who you've heard it from): Bleach will not work on porous or dirty things. I'm sorry, you can't disinfect that cutting board you bought from the thrift store. Or the second-hand drift wood for your lizard's terrarium. We've all done it at one time or another. You're not the first one to dump bleach onto the slobbery dog toys, or litter-encrusted cat box, or into your parvo-contaminated lawn, or spritz it on your ringworm-covered clothes, and think that you killed a few germs. It really doesn't do anything except sometimes change the object's color.And I have more bad news: the same is true for all the other disinfectants out there; there is no such thing as a magic disinfectant that will solve all your organic material problems, no matter what certain brands' marketing implies. (With the possible exception of accelerated hydrogen peroxide and potassium peroxymonosulfate, but even those are only slightly better in the presence of organic matter. Plus they're much more expensive, and have higher health risks to users, than bleach).
But do not despair! Microorganisms don't always need to be killed, they can be PHYSICALLY removed from their hideouts. Washing with detergent, tons of rinsing, hot dry cycles. Don't forget about that overlooked hero, Heat. The most important disinfection process of all, autoclaving surgical instruments, doesn't use any special chemicals at all, just heat.
And sometimes you have to just bite the bullet and throw contaminated things away. If that thing is your yard, digging out the contaminated soil and throwing it away, or burying it in a thick layer of mulch may be a better answer than trying to clean it. Speaking of...
Soiled soil
Yes, it's tough when an outdoor area is contaminated by something nasty and contagious. The most porous and dirt-laden place of all is the same place most likely to be contaminated when you have a sick pet. Most of our worst enemies like parvo, panleukopenia, crypto, and ringworm can live for a long time in the soil. Many pet websites (and a lot of veterinarians) recommend, in addition to other protocols, at least giving a chemical disinfectant a try on your yard if you've had for example, parvo dogs pooping there. I could probably get on board with the idea of giving it a try, just in case it kills a few baddies, except for one problem: if it doesn't work, (and the science tells us it probably doesn't) you won't be able to tell, and you risk getting complacent.
Fabric
Dealing with contaminated clothing is almost as fraught as a contaminated yard. Yes, some bleach products are labeled to disinfect clothing - but in a very limited way: Adding the correct amount to a load of laundry in a washing machine run with cold water that doesn't contain very much organic material. In which case, the action of the detergent and water alone may physically remove more germs than the bleach will kill.
The problem is, I've seen way more people using bleach incorrectly with clothing than any other application. I've observed multiple professionals (veterinarians, certified vet techs, nurses) recommend spraying contaminated clothing with a bleach solution (or other disinfectant, like a quat, usually). This isn't doing jack sh*t. I totally understand you want to feel like you're doing something, but using it this way isn't just useless, it may make things worse by encouraging less caution with the (still contaminated) clothing.
Proper use of bleach: an original mnemomic, just for you!
If you have a non-porous thingy to disinfect, remember to S.W.A.P.
Stay cool! Heat kills bleach (aka, it accelerates the loss of the chlorine ions before they can do their work). Mix with cold water only; if you're going to use in a washing machine, set it to cold; don't use bleach on something hot, like recently washed dishes.
Wash First! -- always pre-clean before disinfecting.
Alone! -- Bleach works best alone. Don't mix it with anything besides water. Unless you're experimenting with re-creating mustard gas, and want to burn your lungs out, then by all means.
Yes, I know they make "cleaner and disinfectant in one" products that had to have passed some efficacy tests before being sold. Buy them if you can afford it, but please, please, don't try to mix your own. Don't try to mix in an essential oil because you don't like the smell of bleach (yes, I once had to stop a volunteer from doing this at the shelter).
Proper dilution! more is not always better. Believe it or not, for some germs, more concentrated bleach doesn't work as well as properly diluted bleach - I suppose it would be as if Super Bleach were all hopped up on Angry Acid and screamed the whole time he ran down the street before attacking the bad guys' hideout - announcing his presence so forcefully would give the bad guys a chance to run away (non-enveloped bacteria link). The perfect dilution of Bleach allows it to punch through the bad guy's defenses with exactly the right amount of force and speed to complete demolish them it before they know what's happening.
OK, that's one of the worst mnemonics I've ever heard, but you get it.
Now, go forth and kill some parvo!
References:
Bleach factsheet from the NIH
Bleach factsheet from the CDC
Cleaning and disinfecting in shelters
UW FAQ on killing parvo
UC Davis FAQ on killing parvo in yards
Resistance mechanisms of bacteria to antimicrobial compounds (PDF)
Antiseptics and Disinfectants: Activity, Action, and Resistance
Bacterial resistance to disinfectants containing quaternary ammonium compounds
How long can different germs survive on surfaces?
Air temperature greatly affects how long crypto can live on the ground: one day vs. 73 days
Image description: a crude cartoon drawing of a anthropomorphized bottle of bleach, labeled "super bleach" and wearing a mask and cape, as it faces off against a blob of brown substance containing anthropomorphized microorganisms, who are frowning at the hero.
