Viruses and AIDS/HIV
Viruses: Definition and Characteristics
A Virus is a non-cellular infectious entity that contains either RNA or DNA enclosed in a proteinaceous coat. Viruses can only reproduce inside living cells by using the host's biosynthetic machinery, making them obligate intracellular parasites. The branch of science that studies viruses is called virology. The word virus is derived from the Latin word venome, meaning poisonous fluid. Viruses lack their own metabolic machinery for synthesizing nucleic acids and proteins, so they depend entirely on the host cell to carry out these vital functions. During replication, viruses may cause disease in the host.
Size Range: Viruses range from 20 nm (parvoviruses) to 250 nm (poxviruses), making them 10 to 1000 times smaller than most bacteria
Filterability: Because of their small size, viruses can pass through porcelain filters that trap bacteria — historically they were called filterable viruses
Artificial Growth: Viruses cannot be grown on artificial media; they reproduce only in living animal cells, plant cells, or microorganisms through a process called replication
Antibiotic Resistance: All viruses are resistant to broad-spectrum antibiotics such as penicillin and streptomycin, which target bacterial cell processes
Replication: Viruses reproduce by replication — a process in which many copies or replicas of the virus are formed using the host's machinery
The concept of viruses evolved significantly over time. In 1884, Charles Chamberland discovered that the agent causing rabies could pass through porcelain filters that trapped bacteria, establishing the existence of filterable infectious agents. In 1892, Ivanowski showed that the tobacco mosaic disease was caused by a filterable agent. In 1935, Stanley crystallized the tobacco mosaic virus and chemical analysis revealed it contained only nucleic acid and protein, confirming that viruses have a simple chemical composition. The first viral disease prevention came in 1796 when Edward Jenner vaccinated a boy with cowpox material, later named vaccination (from Latin vacca meaning cow).
Chamberland (1884): Demonstrated filterable agents by showing rabies passes through porcelain filters while bacteria cannot
Ivanowski (1892): Discovered that tobacco mosaic disease is caused by a filterable agent using bacteria-free filtrate
Stanley (1935): First to crystallize a virus (TMV), proving viruses are simple chemical entities of nucleic acid and protein
Jenner (1796): Pioneered vaccination using cowpox material to protect against smallpox
Prions are the most recently discovered (1983) and least understood infectious agents. They are composed of protein only and contain the information that codes for their own replication, unlike all other organisms which store genetic information in nucleic acid (DNA or RNA). Prions are responsible for mad cow disease and mysterious brain infections in humans. Their protein-only nature makes them highly controversial among scientists.
Protein-Only Composition: Prions contain no nucleic acid, unlike viruses which always have DNA or RNA
Self-Replication: Prion proteins can code for their own replication despite lacking genetic material
Diseases: Responsible for bovine spongiform encephalopathy (mad cow disease) and Creutzfeldt-Jakob disease in humans
Virus Structure and Components
The complete, mature, and infectious virus particle is called a virion. Each virion consists of a central core of nucleic acid (either DNA or RNA), known as the genome, surrounded by a protein coat called the capsid. The capsid is made up of protein subunits called capsomeres, and the number of capsomeres is characteristic of a particular virus. For example, the herpes virus has 162 capsomeres and the adenovirus has 252 capsomeres in its capsid.
Genome: The nucleic acid core (DNA or RNA) that carries the viral genetic information
Capsid: The protein coat that surrounds the genome and gives the virion its definite shape
Capsomeres: The individual protein subunits that assemble to form the capsid — their number is fixed for each virus type
Capsomere Counts in Common Viruses
•
Herpes virus — 162 capsomeres
•
Adenovirus — 252 capsomeres
Viruses vary in their structural complexity. Some have an additional outer covering called an envelope, which is derived from the host cell membrane. When present, the combination of nucleic acid and capsid is called the nucleocapsid, and it is enclosed within the envelope. Viruses without an envelope are called naked virions. Viruses come in several distinct shapes that help in their classification.
Naked Virions: Viruses consisting only of nucleocapsid without any envelope
Enveloped Viruses: Viruses whose nucleocapsid is covered by a host-derived membrane, often containing viral glycoproteins as spikes
Polyhedral Shape: Many-sided geometric form, commonly seen in animal and plant viruses
Helical Shape: Spiral or rod-shaped structure
Icosahedral Shape: Having 20 triangular faces — the most common symmetry among animal viruses
Complex Shape: Viruses with a combination of structures, such as bacteriophages with distinct head and tail
Bacteriophages: Structure and Life Cycles
Bacteriophages are viruses that infect bacteria. They were discovered independently by Twort in 1915 and D'Herelle in 1917. D'Herelle named them bacteriophages meaning bacteria-eaters. Among the best-studied phages are the T phages, particularly T₂ and T₄, which infect Escherichia coli. The T₄ phage resembles a tadpole and consists of a head and tail. The head is an elongated pyramidal, hexagonal, prism-shaped structure containing double-stranded DNA. The tail has a complex structure with an inner core tube enclosed in a sheath, a collar on one side, an end plate on the other, and six tail fibers attached to the end plate for attachment to the bacterial surface. The volume of the T₄ phage is about 1/1000 of its host cell.
Head: Elongated pyramidal, hexagonal, prism-shaped structure containing double-stranded DNA
Tail Sheath: Outer protein layer surrounding the inner core tube — contracts during infection
Tail Core: Inner tube through which viral DNA is injected into the host cell
Collar: Structure on one side of the sheath connecting head and tail
End Plate: Structure at the base of the tail where tail fibers are attached
Tail Fibers: Six attachment structures that bind to receptor sites on the bacterial cell wall
The lytic cycle is the replication cycle in which a bacteriophage takes over the host cell's machinery, produces new phage particles, and ultimately destroys (lyses) the bacterial cell. The phage that causes lysis is called a virulent phage.
Attachment (Adsorption): The phage binds weakly to specific receptor sites on the bacterial cell wall through its tail fibers
Penetration: The tail releases the enzyme lysozyme to dissolve a portion of the bacterial cell wall; the sheath contracts, forcing the core through the cell wall and membrane; viral DNA is injected into the cell while the protein coat remains outside
Synthesis (Multiplication): The viral DNA takes control of the host's biosynthetic machinery and directs it to produce viral components (DNA and proteins)
Maturation: New phage heads are assembled and packed with DNA; tails, collars, sheaths, base plates, and tail fibers are attached
Release: Approximately 200 new bacteriophages are formed within about 25 minutes; the bacterial cell undergoes lysis and releases the new phages to infect other bacteria
Not all phage infections result in lysis. In the lysogenic cycle, viral DNA integrates into the bacterial chromosome instead of taking over the host machinery. The integrated viral DNA is called a prophage and the process is called lysogeny. The bacterium continues to live and reproduce normally, with the viral DNA being passed to each daughter cell during binary fission. The phage responsible for lysogeny is called a temperate phage. Lysogenic bacteria are resistant to infection by the same or related phages. Sometimes the viral DNA detaches from the host chromosome and enters the lytic cycle — this process is called induction.
Prophage Formation: Viral DNA inserts itself into the bacterial chromosome and becomes part of it
Silent Replication: The prophage is replicated along with bacterial DNA during every cell division — all daughter cells carry the prophage
Immunity: Lysogenic bacteria are resistant to further infection by the same or related phages
Induction: The prophage can spontaneously or in response to environmental factors detach from the bacterial chromosome and enter the lytic cycle
Lytic vs Lysogenic Cycle Comparison
•
Lytic: phage DNA takes over host machinery → Lysogenic: phage DNA integrates into host chromosome
•
Lytic: cell is destroyed by lysis → Lysogenic: cell survives and reproduces normally
•
Lytic: ~200 new phages released in ~25 min → Lysogenic: no new phages produced during silent replication
•
Lytic: caused by virulent phage → Lysogenic: caused by temperate phage
•
Lysogenic bacteria are immune to same/related phage infection
Classification of Viruses
Viruses are classified based on multiple criteria: the type of genetic material (DNA or RNA), whether they have an envelope, their overall morphology, and the type of host they infect. Virus morphology and nucleic acid properties are the most important criteria for classification. On the basis of genetic material, viruses are broadly divided into DNA viruses and RNA viruses, and each category includes both enveloped and naked forms.
DNA Viruses: Contain DNA as their genetic material; may be double-stranded or single-stranded; examples include herpes virus, adenovirus, poxvirus, and hepatitis B virus
RNA Viruses: Contain RNA as their genetic material; may be double-stranded or single-stranded; examples include influenza virus, poliovirus, mumps virus, measles virus, and HIV
Enveloped Viruses: Have a host-derived membrane covering the nucleocapsid; examples include influenza virus, mumps virus, HIV, and herpes virus
Naked Virions: Lack an envelope; consist of only the nucleocapsid; examples include poliovirus, adenovirus, and hepatitis A virus
Viral Classification by Genetic Material and Structure
•
DNA, Enveloped — Herpes virus, Poxvirus (smallpox), Hepatitis B virus
•
DNA, Naked — Adenovirus (some colds), Papillomavirus
•
RNA, Enveloped — Influenza virus, Mumps virus, Measles virus, HIV, Hepatitis C virus
•
RNA, Naked — Poliovirus, Hepatitis A virus
Viruses are also classified by their host range and the morphology of the virion. Host range refers to the types of organisms a virus can infect — animal viruses, plant viruses, and bacterial viruses (bacteriophages). Morphological classification considers the overall shape of the virion.
Animal Viruses: Infect animal cells; include both DNA and RNA viruses; may be enveloped or naked; cause diseases in humans and other animals
Plant Viruses: Infect plant cells; many are RNA viruses; rod-shaped (e.g., tobacco mosaic virus) or polyhedral
Bacterial Viruses (Bacteriophages): Infect bacteria; have cubical (icosahedral) or helical symmetry; many have complex head-and-tail structure like T₄ phage
Host Specificity: Most viruses can infect only host cells that possess the specific receptor molecules the virus needs for attachment and entry
Viral Shapes and Examples
•
Rod-shaped — Tobacco mosaic virus (TMV)
•
Spherical — Poliovirus, HIV
•
Icosahedral (20 faces) — Adenovirus, Papillomavirus
•
Tadpole-like (head + tail) — T₂, T₄ bacteriophages
•
Complex — Poxvirus (brick-shaped)
•
Helical (spiral) — Influenza virus, Rabies virus
Human Viral Diseases
Viruses cause a wide range of diseases in humans. The following are some of the most significant viral diseases, their causative agents, and key characteristics.
Smallpox: Caused by poxviruses (DNA, enveloped); ancient disease known since 12th century BC; produces raised fluid-filled vesicles that become pustules and leave pitted scars called pocks; declared eradicated by WHO in 1980
Herpes Simplex: Caused by herpes virus (DNA virus); forms vesicular lesions in epithelial layers of ectodermal tissues; most commonly affects the mouth, lips, and skin
Influenza: Caused by enveloped RNA viruses; widespread disease occurring in epidemic form
Mumps: Caused by paramyxoviruses (large, enveloped, RNA viruses); highly contagious but seldom fatal; about 60% of adults are immune
Measles: Also caused by paramyxoviruses; one of the commonest childhood diseases; develops immunity in victims; adult population is equally susceptible worldwide
Poliomyelitis: Caused by poliovirus (smallest known RNA virus in spherical capsid); occurs mostly in childhood; age of primary infection varies with social and economic factors
Summary of Major Human Viral Diseases
•
Smallpox — Poxvirus (DNA, enveloped) — eradicated 1980
•
Herpes simplex — Herpes virus (DNA) — vesicular lesions
•
Influenza — Influenza virus (RNA, enveloped) — epidemics
•
Mumps — Paramyxovirus (RNA, enveloped) — contagious, seldom fatal
•
Measles — Paramyxovirus (RNA, enveloped) — childhood disease, confers immunity
•
Polio — Poliovirus (RNA, naked, spherical) — childhood, smallest known virus
•
Hepatitis A — HAV (RNA, non-enveloped) — mild, short-term
•
Hepatitis B — HBV (DNA) — transmitted by body fluids, common in Asia/Middle East
•
Hepatitis C — HCV (RNA, enveloped) — often leads to chronic liver disease
•
AIDS — HIV (RNA, enveloped, retrovirus) — immune system failure
Hepatitis is an inflammation of the liver caused by viral infection, toxic agents, or drugs. It is characterized by jaundice, abdominal pain, liver enlargement, fatigue, and sometimes fever. It may be mild or acute and can lead to liver cancer. There are several types of viral hepatitis, each caused by a different virus with distinct transmission routes and severity.
Hepatitis A (HAV): RNA virus, non-enveloped; transmitted through contact with faeces from infected individuals; causes mild, short-term, less virulent disease; vaccine available
Hepatitis B (HBV): DNA virus; very common in Asia, China, Philippines, Africa, and the Middle East; transmitted by exchange of body fluids (blood serum, breast milk, saliva), from mother to child, and by sexual contact; genetically engineered vaccine available
Hepatitis C (HCV): RNA virus, enveloped; transmitted through blood, from mother to child, and by sexual contact; less severe than A or B initially but often leads to chronic liver disease; no vaccine available
Hepatitis D: Delta hepatitis; requires co-infection with HBV
Hepatitis E: Transmitted through faeces of infected person; pig may be a source of infection
Hepatitis F and G: Caused by viruses yet to be fully identified
Hepatitis Quick Comparison
•
HAV — RNA, Naked — Faecal-oral — Mild — Vaccine: Yes
•
HBV — DNA, Enveloped — Body fluids — Can be chronic — Vaccine: Yes
•
HCV — RNA, Enveloped — Blood — Often chronic liver disease — Vaccine: No
Retroviruses
Retroviruses (also called oncoviruses) are single-stranded RNA viruses that have the unique ability to convert their RNA genome into DNA. They are spherical in form, about 100 nm in diameter, and enveloped by the host plasma membrane. Although a few retroviruses are non-specific and can infect any cell, most infect only host cells that possess the required receptor molecules. Retroviruses are associated with tumor production in many animal species including fowl, rodents, and cats.
Reverse Transcriptase: Retroviruses possess a special enzyme called reverse transcriptase that converts single-stranded RNA into double-stranded viral DNA — a process that reverses the normal direction of genetic information flow (DNA → RNA → protein)
Provirus Formation: The viral DNA can be incorporated into the host genome as a provirus and can be passed on to progeny cells during cell division
Oncogenic Potential: Some retroviruses can convert normal cells into cancer cells when the provirus disrupts normal gene regulation
Receptor Specificity: Most retroviruses can infect only host cells bearing specific receptor molecules on their surface
HIV/AIDS
HIV (human immunodeficiency virus) is the most familiar retrovirus and the causative agent of AIDS (acquired immune deficiency syndrome). HIV was first reported in the early 1980s in young patients with symptoms including severe pneumonia, a rare vascular cancer, sudden weight loss, swollen lymph nodes, and general loss of immune function. The virus was identified in 1984 and named HIV in 1986. HIV infects several types of leukocytes (white blood cells) and tissue cells that possess the required receptor. The major cell infected by HIV is the helper T-lymphocyte, which is a critical component of the immune system.
Target Cell: HIV primarily infects helper T-lymphocytes through CD4 receptor sites on their surface
Immune Destruction: As HIV infection progresses, the decline in helper T-lymphocytes leads to failure of the immune system
Secondary Infections: The immunocompromised person becomes susceptible to opportunistic infections and diseases that a healthy immune system would normally fight off
CNS Infection: HIV can also infect cells in the central nervous system
Host Specificity: HIV infects and multiplies in monkeys but does not cause disease in them, demonstrating host specificity
The HIV infection cycle involves four key stages. The virion enters the host cell and is uncoated in the cytoplasm. Then reverse transcriptase uses the viral RNA as a template to create a DNA strand, which is then completed into a double helix. This DNA enters the nucleus and integrates into the host's chromosomal DNA, becoming a provirus. The proviral DNA is then transcribed into RNA and translated into viral proteins. Finally, new capsids assemble around viral RNA and reverse transcriptase molecules, and the new virions bud from the plasma membrane to infect other cells.
Step 1 — Attachment, Entry, and Uncoating: The virion binds to CD4 receptor sites on the host cell surface, enters the cell, and the capsid is removed in the cytoplasm
Step 2 — Reverse Transcription and Integration: Reverse transcriptase converts single-stranded viral RNA into double-stranded DNA; the DNA enters the nucleus and integrates into the host chromosome as a provirus
Step 3 — Viral RNA and Protein Synthesis: The proviral DNA is transcribed into RNA molecules, which are then translated into viral proteins using the host's ribosomes
Step 4 — Assembly and Release: New capsids assemble around viral RNA and reverse transcriptase, and the completed virions bud from the host cell's plasma membrane, acquiring an envelope
AIDS is characterized by severe immune deficiency caused by the progressive destruction of helper T-lymphocytes by HIV. The symptoms first reported in the early 1980s included severe pneumonia, a rare vascular cancer (Kaposi's sarcoma), sudden weight loss, swollen lymph nodes, and general loss of immune functions. HIV is transmitted through intimate sexual contact, contact with infected blood, breastfeeding, and during healthcare procedures. Prevention strategies include avoiding direct contact with HIV, using sterile needles and syringes, and safe sexual practices. An experimental vaccine against HIV began administration in early 2001 in South Africa.
Key Symptoms: Severe pneumonia, rare vascular cancer, sudden weight loss, swollen lymph nodes, progressive immune failure
Sexual Transmission: HIV is transmitted through intimate sexual contact with an infected person
Blood Transmission: Contact with infected blood through blood transfusion, shared needles, or contaminated medical equipment
Mother-to-Child: HIV can be transmitted from mother to child during pregnancy, birth, or breastfeeding
Healthcare Exposure: Healthcare workers can acquire HIV during professional activities involving contact with infected blood or body fluids
Prevention: Avoiding direct contact with HIV, using sterile needles and syringes, safe sexual practices, and screening of blood and organ donations
HIV Transmission Routes
•
Intimate sexual contact
•
Contact with infected blood (transfusions, shared needles)
•
Breastfeeding (mother to child)
•
Mother-to-child during pregnancy and childbirth
•
Healthcare occupational exposure