Viruses are the smallest particles that are only alive inside the host and replicates inside them using host machinery because virus consists only of genetic components and proteins.
They cause disease in all the organisms in animals, plants, bacteria, fungi, etc.
The completely produced infectious virus is referred to as a virion.
The virion's primary function is to deliver its molecule DNA or RNA into the host cell, where its genome is transcribed and translated by the host cells.
Viruses infect organisms and replicate themselves.
It accomplishes this by infiltrating the host's cellular machinery and replicating itself.
However, the host cell normally dies as a result of the procedure.
Examples:
Human Immune Virus (HIV): This causes AIDS (Acquired Immune Disorder Syndrome) this virus mainly targets the white blood cells in humans and deadly disease.
Varicella zoster virus: This causes chickenpox in humans.
Lyssavirus: This causes rabies in dogs, cats, humans, and other animals.