Viruses

Viruses are microscopic infectious agents that can only replicate within the living cells of a host organism. They are considered obligate intracellular parasites, meaning they require the cellular machinery of a host to reproduce and propagate. Viruses consist of genetic material—either DNA or RNA—encased within a protein coat called a capsid. Some viruses also have an outer lipid envelope derived from the host cell membrane.

Viruses vary widely in size, structure, and genetic composition. They infect a broad range of hosts, including animals, plants, fungi, bacteria (bacteriophages), and archaea. Their modes of transmission include air, contact with infected fluids, vector organisms like insects, and contaminated surfaces.

The replication cycle of a virus involves attachment to a susceptible host cell, entry into the cell, replication of viral genetic material, assembly of new viral particles, and release to infect additional cells. This process can cause cellular damage or death, leading to disease symptoms.

Viruses are responsible for many diseases in humans, such as influenza, HIV/AIDS, and COVID-19, as well as in other species. Despite their simplicity, viruses play significant roles in ecosystems and evolutionary processes, facilitating genetic exchange among organisms.

Viral genomes are highly diverse, consisting of single or double-stranded DNA or RNA, and often exhibit high mutation rates. This genetic variability can complicate vaccine development and antiviral strategies. While traditionally viewed solely as pathogens, recent research suggests viruses may also influence immune system regulation and contribute to the genetic diversity of their hosts.

Due to their biological diversity and economic impact, viruses continue to be a major focus of scientific research, aiming to understand their biology, develop treatments, and prevent infections.