How do you get scabies?
Skin to skin contact
Scabies are transmitted primarily through skin to skin contact. Currently, the minimum length of skin to skin contact required to catch scabies is unknown. Mites burrow into the skin and reproduce.
Skin to skin contact: how long does it take?
Though this question is unanswered by science so far, I'll take a stab at it and say it varies; catching scabies like a game of chance, with the odds stacked against you. I am quite certain the it's possible to have very brief - even instantaneous - contact with somebody who has scabies, and catch it. This contradicts much of the scabies information on the net, which states that it takes 10 minutes of contact for scabies to transfer from one human to another. I imagine the actually length of time and probability of transmission depends on a number of factors.
One of the factors is how many scabies a person actually has on them - this is said to be normally in the range of 12 to 25 mites (I think it's probably higher), but can be in the millions in people with compromised immune systems. The more scabies a person has on them, the more likely transmission will occur. Scabies are removed by washing (but not eliminated), so the more a person washes, the less likely they are to transmit them.
There are other ways transmissibility might be affected; imagine yourself as a scabies mite, not with much individual intelligence, but tutored by countless years of evolution's trial and error. When are the best times for you to come up from your burrow under the skin and attempt a transfer on to a new host? The ideal time, I would think, is when your current host is sweating. You are a moisture loving creature, so sweat protects me; furthermore, since sweat, a liquid readily comes of on to another host's skin, it might actually aid you in the mechanical transfer to another host by allowing you to transfer within a drop of sweat. Finally, if you have the ability to sense day and night, then night, when your host animal tends to have the most skin contact with other hosts, would be the best time for you to venture forth. Interestingly, scabies tends to itch more at night. Finally, add together sweating and nighttime, and you get the most ideal situation for transfer: you guessed it - sex!
Trough the environment
Infection from the environment is much less likely than skin to skin contact. Mites are attracted toward heat and smell, and can move over skin at about 2.5 cm per minute. How long a mite can survive off-host depends on humidity; cool, humid conditions extend their survival time off-host. At normal room temperatures, a female mite can survive up to five days off-host, but apparently only retain their ability to infect for the first one to two days.
Some thoughts about the environment and transmissibility.
If extensive research on catching scabies from the environment is ever done, then I imagine the results would tell us that some sites are much better at transmitting scabies than others. What sites might be good or bad for transmission? - they might be different from what you'd expect. My guess is that the most important which factor which determines a site's transmissibility is it's texture. Scabies mites are very small - less than half a millimeter in length - and movement over rough surfaces has got to be difficult for them, so rough surfaces would impair their ability to move toward a host. Furthermore, fibers on rough surfaces, such as carpets and upholstery, would tend to block direct skin contact with mites, making it more difficult for transmission to occur. Possible evidence to support this idea, is the fact that scabies aren't generally found on hairy parts of the body. Smooth surfaces, such as countertops, and smooth floors, would seem to be much better surfaces for transmission that rough ones. An ideal surface would be a smooth damp surface - perhaps a toilet seat in a humid bathroom.