Dr Chia has been doing all these studies because he is convinced that enteroviruses are involved.
I see your point.
The study in question was not by John Chia (the study was conducted in the UK in 1993), but yes, I guess with the emphasis on enterovirus research in the UK at that time, there is a possibility some placebo might be involved. But again, that just calls for better controlled studies.
the common misconception that causation in disease is just genes and environment
Are there any other potential causal factors of disease other than genes, epigenetics, and environmental factors?
The basic concept of cause and effect implies that every disease must have at least one material factor that causes it. Diseases cannot just happen without a material cause, unless one believes in things like divine intervention, or bad karma.
If we look at environmental factors, these include:
Chemical toxins (man-made or naturally-occurring).
Various radiations (UV light, cosmic background radiation, etc).
Infectious pathogens (viruses, bacteria, fungi, protozoa and archaea).
Lifestyle factors like diet, exercise, and stress.
These environmental factors may affect a person directly, or they may have affected the mother during pregnancy, which may alter the development of the fetus (for example, cytomegalovirus or rubella infection in pregnancy are risk factors for autism).
So when searching for the causes of disease, that cause surely must involve one or more of those factors.