Causes
You can get scabies when you come in close contact with infected people or share personal items, such as beddings. These are all secondary scabies causes. The primary cause of scabies is an infestation with S. scabiei mites.
These eight-legged mites burrow into the skin and create a tunnel where it lays eggs. The eggs hatch, crawl up to the skin surface, mature, and spread to other areas of the skin, clothing, or people you are in close physical contact with. These mites are microscopic so you cannot see them.
There are several species of scabies-causing mites. They are host-specific [9], which means those that affect humans are different from those of animals. Human contact with mites responsible for animal scabies may result in an allergic reaction, but it does not typically cause scabies.
The mites that cause human scabies are dependent on humans for their survival. They need to live in the skin of a person to feed and survive, but they can live outside a human host for up 2 to 3 days. On average, a person who has scabies has at least 15 to 20 mites [10] living in their skin.
Conclusion
A previous infestation does not make you immune to this skin condition. If you have had scabies before, a subsequent infection will trigger the symptoms much faster. This is because your immune system has learned to recognize it. The best treatment for scabies is to never have it. While the condition is usually treatable, it may become resistant to standard treatments sometimes. This makes it significantly more difficult to cure.
References
[1] http://www.who.int/lymphatic_filariasis/epidemiology/scabies/en/
[2] http://www.cdc.gov/parasites/scabies/treatment.html
[3] https://www.medicalnewstoday.com/articles/16961
[4] https://www.mayoclinic.org/diseases-conditions/scabies/symptoms-causes/syc-20377378
[5] https://www.mayoclinic.org/diseases-conditions/scabies/symptoms-causes/syc-20377378
[6] https://www.medicalnewstoday.com/articles/16961
[7] https://www.medicalnewstoday.com/articles/16961
[8] https://www.medicalnewstoday.com/articles/16961
[9] https://www.mayoclinic.org/diseases-conditions/scabies/symptoms-causes/syc-20377378
[10] https://www.medicalnewstoday.com/articles/16961