There are a number of tick-borne diseases that are serious for humans -- notably lyme disease. But it's possible that the microorganisms that cause these diseases in humans are beneficial for the ticks. At least, that's what this summary of a recent study seems to suggest. (summary from Science CiteTrack):
Physiology: Protecting Against Cold
Even during the Antarctic winter, parasitic ticks of birds remain active. These arthropods spend much of their lives clinging to vegetation and waiting for warm-blooded prey to pass within reach. Neelakanta et al. wondered whether ticks possess any special capacity for surviving the environmental extremes to which they are inevitably exposed. The black-legged tick Ixodes scapularis serves as a vector for the bacterium Anaplasma phagocytophilum, which causes disease in humans as well as in a range of wild mammals. Heavy loads of bacteria in the ticks correlated with good winter survival and high activity of the arthropods. It appears that the bacterium stimulates the production of antifreeze glycoproteins in its host, suggesting a mutualistic arrangement. Antifreeze proteins coat the surfaces of nucleated ice crystals and reduce the rate at which they grow.
These findings are consistent with the idea that some human pathogens may have originated via opportunistic invasions from microorganisms that were originally invertebrate symbionts.
J. Clin. Invest. 120, 3179 (2010).