The name says it all. Leptospira is a spirochete. They look like corkscrews, and much like other spirochetes such as the lyme disease pathogen and syphilis pathogen, the little fuckers will corkscrew into your cells, hiding from the immune system. This also makes it hard for antibiotics like doxycycline to get into the "hosting" cells. Then add in the normal mutation rates in terms of outer cell wall sugars and protein shapes etc and vaccines are not as useful. So they hide from the immune system and they mutate too fast.