'There are a truly huge number of unsolved problems in biology for which the approach and tools of the statistical physicist might prove relevant,' so says Gene Stanley of Boston University. With the introduction of a lively section to Physica A last July called 'Biological applications of statistical physics' for which Stanley is the editor, we are beginning to see emerging solutions in problems such as HIV, biological aging, immunity, evolution, neural networks and much more.

The aids battle
Sorin Solomon of the Hebrew University and colleagues Henri Atlan of the Hadassah Hebrew University Hospital, Irun Cohen of the Weizmann Institute and PhD students Uri Hershberg and Yoram Louzon, have been applying statistical methods to the study of the three-stage dynamics to the fatal disease AIDS, which is caused by the human immunodeficiency virus (HIV).

On initial infection, the immune system attacks HIV and almost wipes it out. But viral particles remain in the body existing at a low level of expression. Each new variant that emerges is again almost wiped out. But, ultimately the viral forces overtake the battling immune system, crippling it and the patient ultimately dies from a secondary infection.

Solomon's team has used a microscopic simulation based on Monte Carlo statistics to follow an HIV infection. Although the immune system can 'win' a battle against a new infection - within days - the timescale over which the war is fought is much longer. After each battle, a few viral particles escape annihilation, emerging as new strains. 'As time passes, the number of mutations generating new viral strains increases,' explains Solomon, 'While the strains with large populations are detected and destroyed totally (except for eventual mutants) by the immune system, the strains with very few individuals remain undetected (as long as their population remains small).'

After several hundred such battles, the number of small 'undetectable' strains (originating in the mutants from the past destroyed large populations strains) become very numerous, he says. Although each is too small to be detected, they are numerous enough in total to destroy a large number of immune cells. At this point one enters the third phase of the disease: HIV infection becomes AIDS. [Physica A 289(1-2) 178-190] Solomon points out that their study is already being confirmed experimentally by independent research