system
Given the following post, respond as if you're commenting on Reddit:
human
eli5: What is the goal of bacteria or viruses that make humans sick? Why do they weaken their host?
gpt
They all "want" to live, divide and spread. They basically use your body as food and a living ground and a factory (viruses). Often the symptoms of a disease are just a side effect of them using your body and it fighting back (e.g. pain, fever), but sometimes the symptoms help spreading them (sneezing).

Evolution plays a big part here:

Over the long term, it is against the interest of pathogens (bacteria, viruses, etc.) to kill or damage its host body, since a dead human does not move around, limiting the chances to spread to other humans. We see that usually diseases become less damaging the longer they have infected humans (decades to millennia). AIDS viruses already have become less deadly. Ebola on the other hand is a very deadly disease, but quite rare, because people die before they can spread it a lot.

On the other side is the common cold. Theses viruses are so successful, because they only make you sneeze a lot, but leave you enough energy to still go everywhere. They are the most "successful" disease, and due to their adaptation to the body, only make you sick a little bit.