A possible solution to the chronic shortage of allografts is xenotransplantation, the use of tissue from an animal donor. Most experts believe that the pig will provide the most suitable solid organs for use in human beings. Although porcine organs are rapidly rejected by a process called hyperacute rejection (HAR), there is hope that several novel therapeutic strategies, already tested in animal models, will overcome this hurdle in patients. Successful clinical trials of these strategies, expected within the next few years, may herald the era of clinical xenotransplantation. However, there is increasing evidence that other barriers, both immune and non-immune, might exist to limit the survival of xenografts beyond the HAR phase. New strategies to overcome these barriers will be needed if long-term xenograft survival equivalent to, or better than, that of allografts is ever to be achieved.

Predicted numbers vary widely but the most authoritative estimate is that about 6950 cases of BSE will occur in cattle in the UK during 1997-2001 if new infections via feed have ceased as expected and if 10% maternal transmission occurs in the last half-year of the maternal incubation period. This assumes no culling or premature slaughter. Agreed cull strategies would reduce these numbers considerably and accelerate the observed rate of decline of the disease, but there is no scientific necessity for any cull. Bovine products that were banned as specified bovine offals in 1989/90 and as specified bovine materials by successive legislation are now excluded from the food and feed chains. As this second of the two-part article on BSE shows, there has been slippage in some of our control measures, but public and animal health are adequately protected if the legislation that has been evolved is enforced along present lines. Meanwhile, for human beings, bovine milk is safe to drink and beef is safe to eat.

Bovine spongiform encephalopathy (BSE), popularly known as "mad cow disease", was discovered in 1986 and has accounted for the deaths of over 165,000 cattle in the UK (by the end of January, 1997) with about 34,000 (mainly dairy) herds involved. The syndrome in the cow includes changes in posture and temperament, apprehension, and loss of coordination. There are many parallels with scraple in sheep, with similar neuropathological changes in the hindbrain that give it a spongiform appearance under the microscope. The facts have been broadly reviewed in The Lancet in 1990 and 1993, and in much more detail elsewhere. In a two-part article, the first of which appears here, we now summarise recent developments.

Three new human herpesviruses have been recognised in the past decade, and add further to our knowledge of human diseases with potential viral aetiologies. These viruses can be included with the other known human herpesviruses found in normal body secretions, particularly saliva. HHV6 and HHV7 have been associated with febrile illnesses and the childhood disease, exanthem subitum. HHV8 seems to resemble Epstein-Barr virus in its possible transforming properties and poses challenging questions for researchers directed at determining its potential role in lymphomas and Kaposi's sarcoma. Research on these herpesviruses can provide valuable new insights into virus/host relationships and mechanisms involved in replicative and latent stages of virus infection.

The era of the "classical antibiotic" may be over. The emergence of resistance has seen to that. Yet no truly novel class of antibacterial agent has come on the market in the past 30 years. Currently there is great interest in peptide antibiotics, especially the cationic peptides. Thousands of such molecules have been synthesised and just a few are entering clinical trials. Because they kill bacteria quickly by the physical disruption of cell membranes, peptide antibiotics may not face the rapid emergence of resistance.

Ionising radiation, in high dose and with acute exposure, is a factor that has been implicated in leukaemogenesis, but what is the evidence for leukaemogenesis and exposure to diagnostic X-rays, to natural terrestrial or cosmic ionising radiation, to electromagnetic fields, or to nuclear energy? Why is population mixing and infection a possible explanation for the clusters of childhood acute leukaemias around the nuclear processing plants of Sellafield and Dounreay? These questions, as well as how chemical agents, including therapeutic substances, might contribute to leukaemogenesis, are discussed in this last article in the leukaemia series.