Viruses are considered inferior micro-organisms and are incapable of multiplying outside of the host’s cells that they infect (so most of them can only withstand very short time outside, but there are exceptions, such as hepatitis B virus (HBV)!

General properties of viruses include:

1. Very simple organization: a genomic core (DNA or RNA) and external shells (one or more)-usually capsid and sometimes viral envelope – which are formed of glycoproteins called viral antigens.

2. Multiplication of viruses is different from that of bacteria: viruses synthesize multiple copies of the original model, and this process is called viral replication.

3. Viruses are insensitive to antibiotics – so you never treat a viral infection with antibiotics but only with antiviral medications; they act on the principle of blocking the various stages of virus replication, from entering the cell and to the exit and infection of other susceptible cells.

4. The size of viruses is 10-100 times smaller than bacteria, so you can not see them at the usual optical microscope (the one you can see in different laboratories) but only to the electronic microscope, which is expensive and harder to use; so many modern methods are used to diagnose viral infections, but usually not microscopy!

Reference