system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How exactly does patient zero for the flu contract the disease? Is it an outside source or a mutation?
gpt
I think what you are asking is how flues and other contagious diseases originate? Well, the origin of illnesses can be divided into two major categories: heirloom and zoonotic. The former are diseases "handed down" by our primate ancestors which we inherited as we evolved. The latter are diseases that we "caught" from our close proximity to other animals, accounting for aproximately 70% of all human disease. The common flu for example, is zoonotic, we did not initially have this disease as a species, but rather we contracted it from close contact with bird species over time. The cause of the common flu, influenza, was a bird disease, and highly mutative - it changes characteristics often. Here's how it likely first transmitted to humans: 

Humans started agricultural society and began having close contact with each other, and the attraction of peridomestic animals like birds and rats occurred due to high density of food. Humans have greater contact with birds all of sudden, as well as influenza - a bird disease. During all this time of contact, the disease is mutating into many different varieties, eventually one is able to be transmitted to a human as well as cause him illness. It has reached the animal-to-human transmission level. However, it is not yet human-to-human transmissible, so a epidemic does not occur. Over time, many of these animal-to-human jumps are made, until eventually a mutant variant emerges with characteristics that allow it to be human-human transmissible! Given a high enough population density, an epidemic emerges. 

In conclusion, there were probably many "patient zeros" who had animal-human transmissible varieties of the disease, and maybe a few with human-human. But at some point, there was one who was able to spread the disease to a high population density, say a person who is sick and sneezes all over food in a market vs. someone who gets sick and dies alone in the forest. The former can be considered "patient zero" - where the epidemic started. In answer to your question, it is both from an outside source and a mutation.