In New Zealand, the possum is recognised as the main cause of Tb infection in cattle and deer herds. 

Behavioural observations show, that when infected, sick possums often appear during daytime on farmland, where they attract the attention of inquisitive cattle and deer.  This allows Tb to pass through respiratory infection.

In addition, however, possums are the primary species able to maintain the disease in the wild throughout most of New Zealand. This is known as being a maintenance host.
Possums appear to be highly susceptible to bovine Tb and because it is spread by close contact between animals, possums can maintain the disease in the wild due to their high population density and the amount of social interaction between individuals.

By contrast, other wild animals that have Tb do not appear able to maintain the disease within their own populations. Instead, the disease is maintained primarily through the predation of other infected animals. These are known as spillover hosts.

The control of possums is doubly important. Not only will reduced possum numbers reduce the likelihood of interaction between farm stock and infected possums, but it will also mean the collapse of the disease in the wild in those areas where possum numbers are kept at very low levels.