What is a Virus?

Viruses are abundantly found, minute entities that have genetic material enclosed within a protein coat. The proteinaceous material enclosing the genetic material of the virus is called a capsid. The genetic material could be either DNA (deoxyribonucleic acid) or RNA (ribonucleic acid). A virus is too minute to be looked at with naked eyes and therefore visualized with the help of a microscope. They are acellular entities that require a host for processes, such as replication, transcription, translation and multiplication. Plants, animals, and bacteria act as host wherein the virus reproduce. Viruses cannot multiply outside the host's body and thus are considered nonliving outside the host body.

What is a viral infection?

Viruses are capable of causing troubles in the human body and any other organisms such as plants, bacteria and animals after infection. It affects an individual by causing infectious diseases. Viral infection occurs when viruses increase by multiplying within the human body, causing tissue damage. The immune response of the body starts its action to fight against the invading viruses. Simultaneous tissue damage and the immune response is responsible for eliciting symptoms of the viral infection. Viral infection can be transmitted from one individual to another through swallowing, inhaling, infected blood transfusion, or direct contact. COVID-19 (Coronavirus disease 2019) is the most recent infectious disease caused by a virus.

What is the effect of viral infection on the human body?

Healthy cells are affected by viral infection. As the viruses attack a host cell, it releases its genetic material inside the cell. This genetic material takes up the responsibility of producing more viral progenies inside the cell. This virus uses the raw materials from the host to replicate its own genetic material and thus hampers the host cell's machinery to replicate and transcribe genetic material. Viral replication eventually results in cell damage, thus interfering with normal cell functions and processes.

Ways by which viral infections are transmitted

Viral infections vary depending on the type of pathogen and site of occurrences, such as respiratory infection, skin infection, foodborne infections, and sexually transmitted infections (STI).

Respiratory viral infections

Most commonly, viruses attack the respiratory systems such as the nose, throat, airways and lungs. As a result, it causes respiratory tract infections. Respiratory viral infections are transmitted through inhalation of the contaminated respiratory droplets released during coughing or sneezing. The transmission could be prevented by frequent handwashing, covering one’s nose and mouth during sneezing and coughing, avoiding touching contaminated surfaces, and contacting infected people. Some commonly known infections are common cold caused by rhinovirus, seasonal influenza, pneumonia caused by respiratory syncytial virus, measles and COVID-19.

• The common cold is associated with sneezing and coughing, sore throat and headache.
• Seasonal influenza is associated with a runny nose, sneezing, body pain and fatigue.
• Pneumonia or lower respiratory tract infection can be severe in extreme age groups, i.e., infants and older people.
• Measles in children results in cough, sore throat, runny nose, and skin rashes.
• COVID-19 is ravaging the world and is caused by severe acute respiratory syndrome coronavirus 2 (SARS-Cov-2). It causes fever and dry cough. Some infections are life-threatening and could also cause death.

Skin infections

Skin is another organ of the body susceptible to viral attack. Skin infection caused by the virus causes redness, rashes, and bumps on the skin. Such infections are caused through skin-to-skin contact, while some are air-borne. Sharing clothes, towels and swimming pools are some modes of transmission. Common skin infections are cold sores and chickenpox caused by herpes simplex virus-1 (HSV-1) and varicella-zoster virus, respectively.

Food-borne viral infections

Viruses travel through the oral-fecal route when contaminated food is consumed. As the virus invades, it causes food poisoning. Food is contaminated by viral particles excreted in feces, not washing hands, and spoiling food while preparing. Hepatitis A and Rotavirus cause a food-borne viral infection. Hepatitis A attacks the liver and causes jaundice. Rotavirus causes watery diarrhea leading to dehydration.

Sexually transmitted infections (STI)

STIs are transmitted through contact with body fluids and blood. It is most commonly transmitted through polygamous sex and infected blood transfusion. Abstaining from sex, being monogamous, avoiding intravenous drugs, contaminated needles and infected blood transfusion can prevent STIs. Some most commonly known STIs are human papillomavirus (HPV), genital herpes caused by herpes simplex virus-2 and acquired immune deficiency syndrome (AIDS) caused by human immunodeficiency virus (HIV). In AIDS, the immune system is compromised.

Immune response against viral infections

Both innate and adaptive immunity aid in fighting viral infections in a host.

Innate immunity

Rapid onset innate response attempts to inhibit viral infection and eliminate the infected cells. It is the first response of the body against the infection. The response is initiated through the pathogen recognition receptors of the Toll-like receptors (TLR) family. The innate response includes various cells and proteins. Virus-infected cells produce signal proteins called interferons (IFN) that are cytokines, producing warning signals to their neighbor cells.

The α/β IFNs are antiviral proteins responsible for defense action during viral infection. More IFN genes are transcribed by binding to the type I IFN receptor and produce an antiviral state in the body. This state inhibits protein synthesis in the cell and prevents viral replication machinery. IFN induces increased expression of major histocompatibility (MHC) class I molecules that present antigens to innate immune cells.

Innate immune cell such as natural killers or NK-cell is also activated. IL-12 cytokines further promote the action of NK-cell. NK-cell kills the infected cells by producing granzyme and perforin and thus protect against viruses. Apart from NK-cell, leukocytes such as neutrophils, immune cells such as macrophages, and dendritic cells control the viral infection.

Adaptive immunity

Following innate immunity, adaptive immunity is associated with B-cells producing the antibody, and T-cells come into action to fight against the viral infection. Both are directed at different targets and thus have different mechanisms. The antibodies bind to the free viral particles and block their attachment to its infection pathway.

On the other hand, T-cells identify and destroy the virus-infected cells. T-cells are efficient in destroying the virus-infected cells, where viruses replicate within cells and infect other cells without interacting with the extracellular environment. Viral particles presented with MHC-I on the surface of infected cells are acted upon by the cytotoxic T-cells and cause lysis or apoptosis of the infected cells.

B-cells produce antibody responses against the viral pathogen. They neutralize, phagocytose, or activate the complement cascade to eliminate viruses from the host body. Both T-cell and B-cell produce memory cells that produce virus-specific quick, effective, and lasting protective immunity on the subsequent encounters of the same virus.

Viral infections in plants

Virus infecting plants could result in poor quality plant products and initiate with an insect bite. Viruses reach a cell and initiate its action of viral genomic replication and inhibit host cell functions. It soon multiplies and reaches the vascular system and infects roots to leaves. Plants have an immune system that fights against viral infections through gene silencing. The viral gene is diced and silenced. However, such defense mechanisms employed by the plants can also be interfered with by the viruses employing special proteins.

Some of the common plant infecting viruses are the tobacco mosaic virus (TMV) and cauliflower mosaic virus (CaMV).

Viral infections in Bacteria

Bacteria can be either beneficial or harmful. Harmful bacteria are capable of causing bacterial infections. The administration of an antibiotic can combat bacterial infections. However, viruses infect bacteria too, and such viruses are called bacteriophages or simply phages.

As an infectious virus infects a bacterium, it injects its genome inside the bacterium. Once inside the bacterium, the viral genome starts viral replication and switches off the host cell’s machinery. The host bacterium cell is lysed as new virion progenies burst out to infect another bacterium. An antibiotic cannot combat viral infection.

Treatment of viral infections

Vaccines and antiviral drugs boost the immune system in the host and treat viral infections blocking the action of the viral proteins. They help to inhibit the viral action and reduce the symptoms. Vaccines are also administered for the prevention of viral diseases and infections.

In vaccination, a strain or protein of the disease-causing virus is used. Vaccines against polio, measles, chickenpox and rubella are commonly administered vaccines during childhood. Antiviral drugs such as Tamiflu act against influenza and minimize the duration of the symptoms.

Common Mistakes

• The genetic material of viruses could be either single-stranded or double-stranded DNA or RNA.
• Not all viruses are harmful or cause infections.
• Antibiotics are effective against bacterial infections and not against viral infectious diseases.
• Taking an antibiotic during viral infection could result in the destruction of beneficial bacteria in the body.

Context and Applications

The topic is significant for undergraduate, graduate and doctoral courses.
Especially for

• Bachelor of Science in Biology
• Bachelor of Science and Master of Science in Microbiology
• Bachelor of Science and Master of Science in Virology
• Doctor of Philosophy in Virology
• Vaccine Production Against Viral Diseases

Related Concepts

• Microbial Diseases
• Bacterial Infections
• Fungal Infection
• Epidemiology

Practice Problems

Q1: When are viral infections contagious?

(a) Only after the symptoms appear

(b) Before the symptoms appear till they disappear

(c) Before the symptoms appear

(d) Just before the symptoms disappear

Correct option: (b)

Q2: Which one of the following is a viral infection?

(a) Ringworm

(b) Syphilis

(c) Hepatitis A

(d) Strep throat

Correct option: (c)

Q3: Which one of the following is a sexually transmitted infection caused by a virus?

(a) Chlamydia

(b) Syphilis

(c) Gonorrhea

(d) Genital herpes

Correct option: (d)

Q4: Which is the body’s first line of defense against a viral infection?

(a) Innate immunity

(b) Adaptive Immunity

(c) Vaccine

(d) All of these

Correct option: (a)

Q5: A virus that infects bacteria is called

(a) Viroid

(b) Bacteriophage

(c) Virusoid

(d) Capsid

Correct option: (b)