In part this is due to malfunctions of the very complicated human body – this is how many cancers arise. Malfunctions can also be due to poor diet or lack of exercise – a common cause of heart disease.
In part is it simply the struggle for life – an endless competition between replicating species. Microorganisms that infect humans aren’t really trying to cause disease (although some of them produce symptoms in the infected host that help pathogens to spread – this is why the flu virus makes you sneeze). It’s just that they often commandeer the resources of the cell that they infect and this can kills the cell directly or indirectluy if the immune system spots the infection and kills the cell as a rather blunt way of attacking the virus.
Our immune systems are spectacularly effective at fighting pathogens – and do so every second of every day. People born with genetic defects in the immune system have a very tough time staying alive – as do those infected with the AIDS virus, which infects cells of the immune system. They can survive for quite a time but eventually succumb to other infections that they would otherwise be able to fight off.
People get diseases in a number of ways. One way is through organisms like bacteria and viruses some of which use our bodies to get the resources they need to survive and reproduce (flu, AIDS, tuberculosis). Another way is through the normal mechanisms of our body going wrong (e.g., cancer, Alzheimer’s Disease, diabetes, allergies).