Virus

Viruses are submicroscopic parasites. The simplest virions contain a nucleic acid core of single- or double-stranded RNA or DNA, covered by an outer protective shell of protein (capsid). Three-dimensional capsids either have helical symmetry or icosahedral symmetry. They facilitate virus entry and fusion with cell membrane receptors. The genomes of the simplest viruses encode at most four capsid proteins while the genomes of most complex viruses encode 100 - 200 capsid proteins.

Viruses cannot reproduce or replicate without the machinery of a host cell, whether the cell be of an animal, plant, fungi, protist, or bacteria. Once it infects its host, its genetic material can direct the cell to produce more and more viruses. In addition, viruses prime the immune system, initially preventing it from recognizing the presence of a foreign, harmful pathogen in the body. Viruses are dangerous because they cause disease by harming normal cell and body functions. Since viruses do not have the same components as bacteria, antibiotics do not kill viruses. Rather, antiviral medications and vaccines can eliminate or reduce the severity of a viral infection.

Virology is challenging because the virus population is so diverse, even within same families and genera; every virus exhibits high mutation rates in their genomes, which is a significant factor in triggering virus evolution and diversity. Every virus has unique biological mechanisms and properties, some including: different virion structures and genomic structures, different sizes, different host interactions, different pathogenesis, and different replication cycles, and different symptomatic impacts on the host. Due to the diversity within the virus population, one treatment does not cure all viral infections. Some of the existent virus treatments and vaccines are not promising because virus mutation rates are very high and evolution is ongoing.

Right now, viruses are one of the biggest threats to human health. Though they are microscopic parasites, viruses are very "smart" and thus unprecedented in many ways. Each virus has its own method to manipulate normal cell activity, and manifest itself in the body. Their high mutation rates upon entering the host makes antiviral drug and vaccine development difficult. (last sentence should capture why viruses are not easy to cure - do they mutate too much or is it glycosylation)

• Lodish H, Berk A, Zipursky SL, et al. Molecular Cell Biology. 4th edition. New York: W. H. Freeman; 2000. Section 6.3, Viruses: Structure, Function, and Uses. Available from: https://www.ncbi.nlm.nih.gov/books/NBK21523/
• National Human Genome Research Institute. "Virus," https://www.genome.gov/genetics-glossary/Virus
• NCBI. "Structure and Classification of Viruses," https://www.ncbi.nlm.nih.gov/books/NBK8174/#:~:text=The%20simplest%20virions%20consist%20of,on%20the%20prospective%20host%20cell.