x1 displays viral/contagious aspect, spreading through population/network/geography/vel sim x2 or the ability to do so.
x1 is infectious/a contagion/has tendency to spread, in the broadest sense. Infectious laughter, computer viruses, cancer, viral memes (in the original and more modern senses of the word), as well as biological contagions (bacterial, viral, and fungal infections, etc.). See also: "virme".