1) Some infect crucial target tissues e.g. the brain and maternal placenta.

2) All become latent (as circular DNA in the nucleii of ganglia of lymphocytes) of recovered animals. Subsequent reactivation during stress causes disease or tumours (see above).

3) All are cell-associated and can spread between cells by cell fusion. Inactivated vaccines which induce circulating antibody therefore do not work well and safe strong live vaccines which stimulate long term cytotoxic T cells are difficult to make. (DNA vaccines, with Th1 cytokine genes as adjuvants, are the answer if ever licensed).