Introduction to the Viruses: General properties of viruses: 1-They are very small in size, from  m. 2-They contain one kind of nucleic acid (RNA or DNA) as their genome. 3-They are metabolically inert because they do not possess enzyme systems necessary for the synthesis of new viral material (ribosomes). 4-They are obligate intracellular parasites as they replicate inside living cells. 5-They are only seen by electron microscope.

Viral Structure: Each virus particle or virion is composed of : A protein coat  capsid A nucleic acid  core Many viruses are naked but some are enveloped. Viral capsid: It is the protein coat surrounding the nucleic acid. It is composed of small protein subunits arranged symmetrically around the nucleic acid called capsomeres.

Viral nucleic acid and viral Envelope: Viruses contain either DNA or RNA but not both. Most DNA viruses are double stranded, some are single stranded. Most RNA viruses are single stranded, some are double stranded. Virus envelope : Many viruses are surrounded by a lipid or lipoprotein envelopes which may be covered by spikes (glycoproteins).

Virus Symmetry Viruses have three types of symmetry: Cubical symmetry: These viruses resemble a crystal and are called icosahedral virus. Example: adenoviruses. Helical symmetry: In which the particle is elongated. Most helical viruses are enveloped. Example: influenza virus. Complex symmetry: In which the viruses are complicated in structure. Example: poxviruses and bacteriophage.

Classification of Viruses: Classical virus classification schemes have been based on the consideration of major properties of viruses: 1- The type of nucleic acid which is found in the virion (RNA or DNA, single stranded or double stranded) 2-The symmetry and shape of the capsid (Cubic, helical, complex). 3- The presence or absence of an envelope (enveloped, naked) 4- The size of the virus particle. 5- Antigenic properties. 6- Biologic properties, including natural host range, mode of transmission, vector relationship, pathogenicity, and tissue tropism.

Classification of Viruses: DNA IcosahedralComplex Pox viruses Naked Enveloped Papilloma virus Herpes viruses Hepatitis B

Classification of Viruses: a

Viral replication : Viruses multiply only in living cells. Steps of viral replication: 1- Adsorption: Virus attaches to the cell surface. 2- Penetration (Entry): Enveloped viruses: receptor mediated endocytosis 3- Uncoating: Uncoating is the physical separation of viral nucleic acid from the outer structural components. 4- Transcription of mRNA. 5- Synthesis of viral components: This involves the synthesis of viral proteins and viral genomes. 6- Assembly: New virus particles are assembled by packaging of the genome into capsid. 7- Release: Virus may be released due to cell lysis, or, if enveloped, may bud from the cell.

a Viral replication:

Effect of viruses on cells: Many viruses inhibit host RNA, DNA or protein synthesis (or any combination of these). In general, viruses could induce the following pathogenic effects: 1- Cytopathic effect (CPE): The presence of the virus often gives rise to morphological changes in the host cell. These may include: 1-Membrane blebbing 2-Formation of multinucleated giant cells, or fused cells, known as syncytia

n 3-Production of inclusion bodies (accumulations of viral proteins or virions) in the nucleus or cytoplasm 4-Rounding up and detachment of cells from the culture dish. 5- Cell lysis. 2-Latency: Virus genome persists within the infected cells without replication and the survival of infected cells.

a 3- Chromosome damage: As breakage, fragmentation, rearrangement, translocation. 4- Malignant transformation (oncogenesis): Viruses are known to be the etiologic factors in the development of human tumors including cervical and liver cancer. Examples of oncogenic viruses include: - Human papillomavirus (HPV) : Cervix Cancer - Ebstein Barr virus (EB) : Nasopharyngeal carcinoma - Hepatitis B and Hepatitis C viruses (HBV, HCV) : liver Cancer - Human T cell lymphotorpic virus (HTLV): Adult T cell leukemia.

a Cytopathic effects: Left to Right: Cytopathic effect of HSV, enterovirus 71, and RSV in cell culture. Note the ballooning of cells in the case of HSV. Note syncytia formation in the case of RSV.

Clinical examples on viral infection: Skin infection: One of the most common viral infection of skin: Herpes virus infection. Respiratory tracts infection: The most common viral infection of respiratory tracts: 1-Influenza virus infection. 2-Common cold Rhinovirus. Digestive tracts infection: -Gastroenteritis: Rotavirus. -liver infection: Hepatitis A,B, and C.

N Sexually transmitted diseases : The most common viruses are: 1-Genital Herpes. 2-Papilloma virus. Nervous system Encephalitis and Meningitis: 1-Flaviviruses. (Arbovirus). 2-Rabies virus.

Diagnosis of viral infections: In general, diagnosis of viral infection can be performed by: 1-Direct detection: The clinical specimen is examined directly for the presence of : A-Virus particles: Microscopy. B- Virus antigen: Microscopy. C- Viral nucleic acids: Molecular methods. 2- Virus isolation: For cultivation of viruses, the Clinical specimen is inoculated into: A- Cell culture. B- Chick embryo (fertilized eggs). C- Laboratory Animals.

n 3- Serology: -Elevated antibodies can be detected. -The most common methods used in hospitals. -Can be used to differentiate acute and latent infection. -Different methods can be used in serology such as: 1-Agglutination methods. 2-ELISA.