Susto is usually translated as "fright" but a better way to translate it would be trauma. As Wikipedia puts it:
Susto is a cultural illness, specifically a "fright sickness" with strong psychological overtones. Susto comes from the Portuguese, and Spanish word for "fright" (i.e. sudden intense fear, as of something immediately threatening). A more severe and potentially fatal form of susto is called espanto (also from Spanish and Portuguese, meaning terror or intense fright). It is indigenously attributed to "soul loss" resulting from frightful or traumatic experiences.[1] Among Native Indian populations of Latin America, susto may be conceptualized as a case of spirit attack.[2]Culturally stressed adults (women more often than men) are most likely to suffer from susto, although children may also be afflicted. The onset of the disease generally follows a sudden frightening experience such as an accident, a fall, witnessing a relative's sudden death, or any other potentially dangerous event. Research shows that knowledge of the existence of susto is a major contributing factor in improving the condition.
The condition seems to have some vague connections and maybe origins with the stress-induced out of body experience -- that situation one hears described from time to time during a dramatic or upsetting event of "it was like I was somewhere else watching myself!" -- hence "soul-loss" is used to describe it even though a person stays relatively functional in most cases -- just is never quite himself again, until recovery.
These days many susto cases don't include the out of body experience, perhaps because we're more accustomed to stress nowadays than in the times and cultures where the concept was born. But susto does still arise from high stress situations and has a lot of overlap with medical and psychological concepts like depression and post-traumatic stress disorder. Bullying, a bad breakup, being in an accident or witnessing a violent event are all possible causes, as well as lesser traumas like seeing a dead animal or a scary bug. Some people are even said to be struck with susto while still in their mother's womb! Its symptoms can lead to anything from a general sense of disturbance to actually bringing on chronic illnesses and pains.
At any event, upon getting a susto, the remedy is to coax the spirit back into the body, and the sooner it's done the better. A mild case can be remedied with something very simple like a bath in bluing or drinking teas like orange blossom or orange leaf, mint, or a stronger fig-and-vinegar concoction.
For harsher cases, the typical remedy is to go to a healer where a sweeping ritual is performed, often over multiple days. Unfortunately, many people, even when struck with susto, don't live in an area near such healers -- or, unfortunately in some cases, do have suitable healers nearby but find a bit of a language barrier toward working with them. Susto is something of a Latin-American concept and as such, it can be difficult for Spanish-non-speakers to get much information or help for the condition.
I recently came upon the book Rainforest Home Remedies which has a small section on what is called "spiritual illnesses," and susto is included thereamongst. In it, it includes (naturally!) a home remedy for even severe susto. It is summed up as follows:
*Begin with buying some copal incense, and be sure to burn this daily and especially during the performance of all subsequent steps.
*Start with a series of 9 healing prayers said in a row, and repeat these 9 prayers over multiple days -- 3 days at least. The prayer should be said thrice while holding the pulse of the right wrist, thrice holding the pulse of the left wrist, and thrice holding both hands over the head. Really any healing prayer can be used, I should think, as long as it's sincere, but author Arvigo offers the following suggestion, which can be used for yourself or for a person you are assisting: "In the name of God the father, God the son and God the holy spirit, I am the one who prays for susto. I ask God to help me, and I ask for the nine Maya spirits to help me too. I say these prayers nine times to release this susto from the flesh of ______. I have faith with all my heart that this prayer, these herbs, baths and incense, will cure the susto of ______. In the name of God the father, God the son and God the holy spirit." (NOTE: The Apostle's Creed is also another traditional prayer for the cure of susto.)
*Gather some rue, basil and marigold, and use these to make an herbal bath. Take the bath "twice weekly" is as it's put -- in other words every 3 days.
Proceed in this until you feel completely cured -- how long this will take can vary depending on severity of the case and could even require years of work in some cases.
I quite recommend the Rainforest Home Remedies book if you have any more interest in South American folk medicine or in spiritual illnesses and treatment (though be warned -- rue, basil and marigold bath seems to be the recommended cure for just about every spiritual ailment there is.)
Another remedy I've used for susto and other stresses which I can recommend is New Life incense burned about the room, along with Peace Water or 7 Holy Waters mixture applied to the body. New Life is a formula particularly used for unhappy pasts that need to be let go of -- Cut and Clear is another similar formula. (Recipes for both variants are available in The Conjure Cookbook.) 7 Holy Waters is a nice recipe I got from the ever useful Lucky Mojo site, and I prefer it for applications to the body over many other variants -- it's not oily like Peace Water and it has alcohol to disinfect and preserve it (some folks might live near cleaner rivers than I do, but my local source is a semi-stagnant, muddy mess which ever stinks of storm drain and hobo piss -- so I'm glad for the whiskey and cologne in the mix.)
Susto is a miserable condition, but studies find that awareness of it is key to being able to cure it. Hopefully if you find yourself in such a state, some of this will be useful to you.