system
Given the following post, respond as if you're commenting on Reddit:
human
[Aliens] Why did Ripley believe that "If even one of those things get down here it's all over!"

Why did she expect a single xenomorph to be a threat to an entire planet?
gpt
Because a single Xeno in theory could create an egg (e.g. deleted scenes in the first movie) and from there expand to a queen, more eggs...

Imagine a Xenomorph dropping into the desert in say America, low population density and little military presence. Give it 10 months the Xenos will be established, give it 10 years and mankind will be fighting a way on multiple fronts.

It's an exponential growth pattern with the possibility​ of going undetected for weeks at a time being extremely aggressive​ and smart. So yeah... A single Xeno could do a lot of damage if it gets established.