Lyme disease is the most common disease spread by ticks in the Northern Hemisphere. It is estimated to affect 300,000 people a year in the United States and 65,000 people a year in Europe. Infections are most common in the spring and early summer. Lyme disease was diagnosed as a separate condition for the first time in 1975 in Old Lyme, Connecticut. It was originally mistaken for juvenile rheumatoid arthritis. The bacterium involved was first described in 1981 by Willy Burgdorfer. Chronic symptoms following treatment are well described and are known as "post-treatment Lyme disease syndrome" (PTLDS). PTLDS is different from chronic Lyme disease, a term no longer supported by the scientific community and used in different ways by different groups. Some healthcare providers claim that PTLDS is caused by persistent infection, but this is not believed to be true because no evidence of persistent infection can be found after standard treatment. A vaccine for Lyme disease was marketed in the United States between 1998 and 2002, but was withdrawn from the market due to poor sales. Research is ongoing to develop new vaccines.