Question: how does a certain disease develop certain properties that have certain effects because none of them are the same?

Verity Hill answered on 6 Nov 2018:

Great question! There’s a few different answers to this. I’m going to stick to infectious diseases that are caused by bugs (called pathogens), because I don’t know much about non-infectious ones (like cancer or diabetes). I’m also going to stick to viruses because I study those! But there are bacteria people here that can answer based on those I’m sure.
One answer is to do with how the virus interacts with your cells. In order to make more copies of themselves, viruses have to get into your cell so they can hijack your cells machinery. Human cells have different things sticking out of them that the virus has to interact with to get in, just like a key and lock. The type of cell that a virus can interact with affects what the disease it causes will look like because the virus usually destroys the cell when it has finished- for example Hepatitis C virus can get into your liver cells, so the disease Hepatitis C causes bad liver damage. Rabies virus can get into your brain cells, which is why it can affect your behaviour, and also why it almost always kills the person it infects.
The other answer is one to do with evolution, and how viruses adapt to their human hosts (ie the thing that they live in). Viruses ultimately want to make lots of copies of themselves, and then transmit to more people so that they can make even more copies. So different viruses have different strategies to make this happen. This might affect how the disease affects a person, for example norovirus spreads through poo, so it causes extreme diarrhoea because then this often leads to a new patient. Sometimes though, it makes a virus less likely to be harmful because they don’t want to kill their host too quickly and the symptom that leads to a new patient might be very harmful. This is called the “virulence/transmissibility trade-off”, and it is a fascinating and large area of study in evolutionary biology.
The final answer is that some viruses aren’t adapted to humans! Sometimes they do things that don’t help the virus or the person because they haven’t had time to adapt – for example Rabies doesn’t spread between humans because once it’s in your brain it can’t go anywhere, because it’s really adapted to live in bats where it is much less dangerous. (On a bacteria side note: sometimes microbes “get lost” inside your body – for example the bacteria that causes the serious infection Meningitis actually usually lives quite comfortably in your nose without causing disease – it’s only when it gets into your brain that it causes disease!)
Usually it is the viruses that aren’t really adapted to humans that are most likely to cause very serious symptoms kill you because they don’t know how to keep you alive long enough for you to pass it to other people.
I hope this answers your question! I had a lot of fun thinking about the answer, so please if you have other questions about this, go ahead!