It is very rare that only one thing causes one effect so consistently that it is clearly known. It has long been understood that certain compounds or plants have physiological effects if ingested. This is where almost all of our active medicines come from.

However, let’s consider bugs some more:

Strep. Pneumoniea can lead to pneumonia, but not always. Many other factors will influence this. If the immune system is active and sharp, the body can protect itself from many microbes. But knock out t-lymphocytes, as in AIDS for example, and all kinds of things that were previously not “causes” become highly effective at producing disease. So, is the cause of these AIDS-related diseases, like fungal pneumonia, the fungal agent (proximal cause) or the human immune deficiency virus (pre-disposing factor)? Or is it the damage to the skin that enabled the virus to gain entry to the body? Or is it the emotional resistance to using condoms which enables the virus to cross from one person to another? Or the opportunity for unprotected sexual intercourse with more than one person (distal cause)?

When we look at things this way, we see chains or “nets” of cause and effect, with nodes where several factors converge and an effect emerges, or is suppressed, much like constructive and destructive interference between waveforms. (As an illustration of how our thinking influences how we see the world, in the causal network, diseases can be due to the suppression of “good” effects as well as the occurrence of “bad” effects. This implication isn’t apparent from the simple cause-effect model.)

How we think about causes will determine what we do to try to fix or prevent a disease. Indeed, we can go further and ask what exactly is a disease?