Yes and no. Many bacteria can infect another species without causing disease; hence they become carriers or vectors. A number of bacterial (and viral) agents are now jumping their traditional host species, in part due to the rising number of immuno-deficient individuals in the general population. For example a number of Helicobacter spp. have been found in non-traditional host species, leading to the spread in many populations. In many cases these organisms do not cause outright disease but may have an effect on the immune system making the individual more susceptible to other disease causing agents.

Some viral agents when they jump host species can have a devastating effect on the new host species. For example many parvoviruses and their respective hosts have an almost commensal (non-disease causing or normal) relationship. In some cases the virus causes no outright disease, but influences the immune system either up regulating or down regulating all or parts of the immune response. However, when the agent jumps species or infects a naïve population, outright disease and death is often the result until the virus or host species adapts to the infection over time.

While it is true most parasites are host specific due to their life cycle, there are cases (rare) where a parasitic agent will jump species (aberrant host). Generally it is a dead end infection/infestation, but can make life miserable for the affected host.

Pete