A Coggins test (the name is for the man that invented the way to test for the disease) is a blood test for Equine Infectious Anemia. It's an uncurable, contageous disease. Infected horses have to be quarintined for life or euthanized.
The common names for it are "Swamp Fever" or some people refer to it as "Horse Aids" because of the way if destroys the immune system , etc. It's spread by horse-horse contact or biting insects, like mostquitioes (swampy areas). A sick horse can appear quite healthy, hence the need for blood test. Sometimes they get sick and recover, only to eventually get sick again, etc until they succumb-miserable death!
In the US, most states require proof of a negative test (from within the last year) to cross State lines, the test has to be within 6 months for a sales of a horse and most public horse functions/boarding facilities should or do require proof of a current neg. Test to bring your horse on the grounds. There's no vaccination for it or cure, but it's not very common anymore, thanks to the test and the checking of papers.
If you want to read more.... http://www.thehorse.com/viewarticle.aspx?ID=357