If USB flash drive contain any of malware/virus/worm inserted to linux host without being mount (auto mount disabled), does malware/virus/worm will infect host
If it's a USB stick you don't trust, you should always be careful. There's malicious hardware that looks like a USB stick, but pretends to be a keyboard, making malicious inputs. Or hardware which contains condensators that charge up from your USB port and then send a strong electrical pulse into your PC, potentially frying electronics.