Infective unit of a virus is called Virion.

Explanation:
A virion is the complete, infective form of a virus outside a host cell. It is the basic unit of a virus that is capable of infecting a host cell and causing an infection. The term "virion" is derived from the word "virus" and is commonly used in virology to refer to the infectious viral particle.

Structure of a Virion:
A virion consists of several components that are essential for its infectivity. These components include:

1. Capsid: The capsid is the protein coat that encloses the genetic material of the virus. It provides protection to the viral genetic material and also plays a role in the attachment of the virus to host cells.

2. Nucleic Acid: The nucleic acid is the genetic material of the virus, which can be either DNA or RNA. It carries the instructions for the replication and assembly of new virions inside host cells.

3. Envelope (optional): Some viruses have an outer envelope that surrounds their capsid. The envelope is derived from the host cell membrane and contains viral proteins that are important for attachment and entry into host cells.

4. Spikes (optional): Spikes are viral proteins that protrude from the surface of the virion. They are involved in the attachment of the virus to specific receptors on the surface of host cells.

Function and Importance of Virion:
The virion is the infectious form of the virus and plays a crucial role in the transmission and spread of viral infections. It is responsible for attaching to host cells, entering them, and releasing its genetic material into the host cell. Once inside the host cell, the viral genetic material takes over the cellular machinery to replicate and produce new virions.

Virions are also important in the diagnosis and detection of viral infections. They can be isolated from clinical samples, such as blood or respiratory secretions, and their presence can be confirmed using various laboratory techniques, including electron microscopy and molecular assays.

Conclusion:
In summary, the infective unit of a virus is called a virion. It is composed of a capsid, nucleic acid, and may have an envelope and spikes. Virions are responsible for the transmission, attachment, and entry of the virus into host cells, and they play a crucial role in the spread and pathogenesis of viral infections.