The Hopper virus (also known as Hoppers) could infect anything.
In humans, symptoms included wiggling of the fingers, temporary paralysis of the lower half and hallucinations of bats. It could be cured by pinching the ear of the infected, causing them to sneeze the virus out.
The virus could also infect machines and computer systems and would wreck them. If a person touched an infected machine, they would become infected with the virus. A debug script could neutralize the Hopper virus in a computer system.
At Tranquility Spa, Ryan Sinclair and Bella were both infected by the virus, Ryan from an infected vending machine. The Thirteenth Doctor removed the virus from Ryan, but realized that it had infected the spa's computer systems, disabling the teleporter and ionic membrane. The Doctor managed to neutralize the virus with a debug script and restore the membrane. It was later revealed that Bella had in fact infected the spa's systems with the Hopper virus as part of her plan to destroy it. Bella attempted to infect the systems again by putting the virus where the Doctor couldn't isolate it but one of her bombs blew out the mainframe before she could.
Later, in order to restore the teleporter, Nevi needed syrillium 4 while they only had syrillium 3. The Doctor remembered that syrillium 3 would mutate into syrillium 4 when attacked by the Hopper virus and provided Nevi with the virus that she had removed from Ryan. Sylas was able to use the virus to create the fuel that they needed to get himself, Nevi and Team TARDIS off of Orphan 55. (TV: Orphan 55)