system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why are viruses like the measles so dangerous? Wouldn't the virus want to keep the host alive to help it spread?
gpt
Most people who get measles dont die from it. Measles are dangerous because it can induce lethally high fevers.  But, generally, microscopic organisms like  viruses are built to reproduce in a given environment (like the human body) rather than travel through different environments.  Their ability to spread from host to host is often incidental, such as through a transfer of bodily fluids.  Also, viruses mutate frequently, so the "successful" viruses we know about usually spread by airborne means or skin to skin contact.  There have probably been thousands of lethal viruses that only killed one person then died out with them because they had very specific or no means of transmission.