The field and laboratory evidence incriminating nymphal Ixodes dammini as the main vectors of Borrelia burgdorferi is substantial. Furthermore, other members of the Ixodes (Ixodes) ricinus 'complex', including I. ricinus, I. persulcatus, I. pacificus, and I. scapularis, are competent vectors of the Lyme disease spirochete. Although ticks in other genera are also naturally infected with B. burgdorferi, experimental evidence suggests that Amblyomma and Dermacentor ticks are inefficient vectors of these spirochetes. Current research on the kinetics of B. burgdorferi growth within ticks demonstrates that Lyme disease spirochetes are dramatically influenced by physiological events during the tick's life-cycle.