Genomes and protective structures
A virus particle generally contains a genome and a protein capsid, sometimes with an envelope. Genomes may be DNA or RNA and differ in strand organization and segmentation. Particle structures protect genetic material and participate in host interactions. Morphology, sequence and infection studies provide complementary evidence for comparison.
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Reproduction within host cells
Within a suitable host cell, viruses use or alter molecular processes to copy genomes and make components, followed by assembly of new particles. Requirements differ among viruses. Contact with an organism does not guarantee reproduction in every cell, so host range and tissue conditions are central to understanding life cycles.
Diversity of viruses
Viruses infect animals, plants, bacteria and other hosts, forming varied relationships. Disease research emphasizes damaging interactions, while virology also investigates replication, evolution and cellular processes. Antibacterial drugs target different structures and processes, reflecting important biological differences between viral and bacterial infections rather than a simple distinction of size.
From host responses to molecular evidence
Identifying a virus connects genetic, particle or other laboratory evidence with processes in a host. Symptoms alone often cannot distinguish pathogens. Samples, culture conditions and detection methods shape observations. Identifications such as HIV opened further investigation of transmission, disease mechanisms and interventions rather than settling every question at once.