Viruses can spread themselves, without the knowledge or permission of the workstation users, to potentially large numbers of programs on many machines. Unlike most other programs, viruses are specifically designed to spread themselves.
Viruses can often spread without any readily visible symptoms.
When a virus is started on a workstation, it can run any instructions that its author chooses to include. Depending on the motives of the virus author, a virus can contain no intentionally harmful or disruptive instructions. Or, it can cause damage simply by replicating itself and taking up scarce resources, such as hard disk space, CPU time, or network connections. You cannot rely on strange behavior to find viruses. The most reliable way to find viruses is to use competent anti-virus software.