system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5 Why is a cross (+) normally the signal for first aid?

Symbol, not signal.
gpt
The red cross on a white background was standardized as a symbol for first aid on battlefields by the First Geneva Convention back in 1864.

It was formed by reversing the color pattern in the Swiss flag. By making the flag primarily white, it was associated with white flags of peace on battlefields. By making the flag resemble the Swiss flag, it was associated with the firm neutrality of Switzerland. It was basically a big flag saying "we're just here to save lives, don't shoot us."