Question

how are the threats spread through external storage device and internet​

Answer 1

computer worm is a standalone malwarecomputer program that replicates itself in order to spread to other computers.[1] Often, it uses a computer network to spread itself, relying on security failures on the target computer to access it. Worms almost always cause at least some harm to the network, even if only by consuming bandwidth, whereas viruses almost always corrupt or modify files on a targeted computer.

Many worms are designed only to spread, and do not attempt to change the systems they pass through. However, as the Morris wormand Mydoom showed, even these "payload-free" worms can cause major disruption by increasing network traffic and other unintended effects.