Our vocabulary changes as we learn more. The old words about cleanliness were insufficient - or imprecise - when we want to talked to quantify how clean something needs to be or how much bacteria was on it.
The old words were sanitize, disinfect, and sterilize - in that order of ascending cleanliness. To sanitize means to clean something to the eye. A visual inspection should not reveal substantial dirt or unwanted material. This is how janitors clean toilets and how health inspectors generally evaluate commercial kitchens. To disinfect means to remove pathogens that can cause disease in humans or other mammals. It can only be verified with a scientific test, but industry has generally agreed that cleaning a surface with a liquid or gaseous substance designated a disinfectant counts. That is not foolproof and there can be places where the disinfectant material does not reach but as long as the facility manager makes a good faith effort, the surface or item is considered disinfected.
Bacteria are relatively easy to deactivate. Other pathogens are harder to kill - viruses, some fungi, and particularly bacterial spores which nature has designed to survive extreme conditions. Put a metal medical instrument in a pot of boiling water for a few minutes and you destroy the bacteria, but spores may survive. That’s why even hotter temperatures were required. Sterilization is the eradication of all pathogens.
Why do we need sterilized items and rooms? For the most part we don’t need sterile rooms. Clean rooms used in some industries have standards for particulate count but microbial populations are usually not measured. A clean room at a semiconductor fab is not sterile.
Is a hospital operating room sterile? No, it is occupied by human beings with bacteria on their skin. (Typical bacteria count is 100 to 10,000 per square centimerer of skin). But the operating team can be reasonably sure the scalpel is sterile.
A person with a normal immune system and no open sores should have no problems with anything that has been disinfected. However, when doctors work inside the patient’s body, they want to use equipment that has been sterilized.
The more recent discovery of prions as a vehicle for disease transmission has complicated things. Like viruses, prions are not really alive in the biological sense, but in the right situation they can reproduce and cause illnesses in their hosts.
Doctors' offices, veterinarians, dentists, hospitals, and mortuaries all are concerned with cleanliness.
How do you know if an item or surface or floor has been disinfected or sterilized? Although you could take a sample and examine it for microbes, this is not practical in most cases. Managers usually rely on administrative controls - they keep a history of the item. It’s largely a matter of keeping track of history and knowing what cleaning process has been executed and when the item was cleaned.
Autoclaves are meant to sterilize equipment - lab glassware, scalpels, etc. And also to render medical waste harmless. We have procedures for validating autoclave operation.
But a new paradigm has arisen in our test-happy world. In an effort to be more precise about level of cleanliness, industrial hygienists have developed a new system: Level 1, Level 2, etc.