Some modern humans carry immune genes that originated in Neanderthals and a related species.
It makes humans very nervous for us to carry them here so we left them behind.
No human can carry that burden on their shoulders.
We Earth-born humans have carried the burden of war all through our history.
The kids have been developing those diseases out of weakness and from the reservoir of infections we humans always carry with us.
In 'Fahrenheit 451', a group of humans will carry on and rebuild society.
"Sixty humans couldn't carry the Cloudmaster away on their shoulders."
Consequently we can assume, at this point, that humans do not carry the disease.
They carry one plant at a time, less than the 2 or 4 that humans carry, but the robots don't need to take breaks.
I have wondered why more humans don't carry memory pouches.