“And I will smite the inhabitants of this city, both man and beast: they shall die of a great pestilence.”

Jeremiah 21:6

Why do diseases make us ill? Why do some diseases kill us outright? As with many biological questions this is really two questions. Firstly, “What chemical and biological processes are happeningin our body that makes us feel ill?” Secondly, “What evolutionary gains does a disease get by making us ill?” These two questions are asking about different things known as the proximate and ultimate answer. The proximate question is looking for the mechanism that makes something happen. The ultimate question is asking why that mechanism has evolved in the firstplace.

Why are humans so intelligent? Here again we have a proximate and ultimate question. Proximately, humans are intelligent because we have large brains. Our brains fold in a way that allows more connections to be made and this also increases our intelligence. You can find the answer to this proximate question by finding a willing volunteer, cutting their head open and having a look inside (or maybe looking in a text book). For the ultimate question, we have to look into our past and try to understand whether higher intelligence perhaps allowed humans to hunt and forage better? Or maybe it helped us live in groups, which meant we could fight rival groups better? The point is, there are often two questions hidden in a single biological question.

So then, why do diseases make us ill? I am most interested in the ultimate question here because it is a paradox, and that’s when science gets exciting. It is a paradox because diseases are spread by their human host. If the human dies, or sits in bed with a lemsip, they are not out and about spreading the disease to new hosts. If a new disease arrives in a country, infects one person and promptly kills them, that is the end of the disease. If instead the disease arrives in a country and keeps the one host alive, the infected person wanders around, sneezing near people, kissing their partner(s), shaking hands with work associates etc. etc. The disease spreads. 

Only diseases that keep their hosts alive will spread. All the diseases that have ever infected just one person are gone and forgotten. So, this is the paradox. We see many diseases that kill their host quite quickly, and a few that kill their host very quickly. The best thing a disease can do however, is never kill its host (this is called low virulence). So what’s going on?

The classical answer for this problem is that highly virulent diseases are just less evolved. Diseases like colds that do not kill their host have had a long time to evolve so they have evolved low virulence. More deadly diseases such as malaria must therefore have starting infecting humans much more recently. However, the very high speed of disease evolution makes this theory quite unlikely. However, slightly turned on its head, this theory becomes useful. Maybe diseases aren’t adapted to living in humans at all. It seems likely that many diseases that kill humans are not really ‘human diseases’. The Ebola virus is a good example. In humans it is fatal. However, it is likely that it is not a human disease at all, but a bat disease. It is not lethal to bats, and so is a successful disease. On occasions however, it gets spread to humans. It is not adapted to living in humans and so ‘accidentally’ kills its host.

Another theory comes from the idea of trade-offs. Trade-offs are very common and very important in biology for much the same reason as they are important to humans; we have limited resources. I like reading, but I also like walking in the Peak District. However, I have limited time. Going for a walk indirectly reduces the amount of reading I can do. So I try to find the right balance. I still don’t read as much as I would like, or do as much walking as I would like, but I do the best I can. What then are the trade-offs that affects virulence? 

Transmission is how well a disease can spreaditself to other hosts. It depends on many things such as how many copies a disease makes of itself in its host. The more malaria bacteria in the blood,the more likely it will be picked up and spread by a mosquito. However, these bacteria need nutrition, and so the more bacteria, the more of the bodies resources they use and the more ill the host becomes. Here then is a trade-off between transmission and virulence. The balance found by diseases depends on many things such as how densely populated the area is and whether the disease is spread by contact, by animal vectors or some other means of transmission. 

This then explains why some diseases are quite fatal, whereas others are benign. It is an ultimate explanation. I don’t know much about the specifics of how malaria kills people or how the common cold avoids killing its host, but I know that they are both struggling with a trade-off between transmission and virulence. This trade-off is (ultimately) why some diseases make us slightly ill and some diseases kill us very rapidly.