1 Introduction

A virus is an infectious agent that is submicroscopic and replicates inside the living cells of an organism. It infects various forms of life including plants, animals, and humans. The study of viruses is called virology. Viruses lack the cellular organization that characterizes life [1]. They have no nucleus or other organelles and no cytoplasm. Viruses must enter a living host to multiply. They contain a nucleic acid (DNA or RNA) and proteins. The nucleic acid occupies the core region of the virus. Some viruses contain DNA (deoxyribonucleic acid), while others have RNA (ribonucleic acid). True cells have both types. Viruses are extremely small. Some are only 20 nanometers in diameter. Knowledge about their structure became available by using the electron microscope [2,3,4,5]. Viruses exist in a variety of forms such as spheres, cubes, rods, etc. The nucleic acid of a virus is enclosed within a shell made of protein. This shell or jacket is called a capsid. It may be rod-shaped or a more complex form, depending upon the type of virus. Capsids are made of many protein subunits called capsomeres. For a simple example, tobacco mosaic virus is a plant virus that has RNA and a helical structure. See Fig. 1.1 [6]. This virus infects a wide variety of plants, especially tobacco [7]. It causes the plant’s leaves to have patterns such as mosaic-like mottling and discoloration.

Fig. 1.1
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The tobacco mosaic virus contains a strand of RNA coiled in a helix of protein subunits

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Some animal viruses and others have a complex structure in which their capsid is surrounded by a membrane envelope. This envelope is covered with spikes that project outward from the surface. The influenza virus has an outer envelope studded with glycoprotein spikes [8,9,10]. Its genome (set of genes) consists of eight single-stranded RNA segments (each of which is wrapped in a helical capsid). This virus generally has a spherical shape. Figure 1.2 shows the structure of a coronavirus with its harmful spikes [11]. It has the RNA genome too. The display is of the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). This virus is contagious and can affect various parts of the human body [12,13,14,15,16,17,18]. It is primarily spread between people in close contact and by droplets from coughs, sneezes, etc. of individuals with the virus.

Fig. 1.2
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This figure shows the structure of a coronavirus with its harmful spikes

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Viruses can’t multiply on their own. They replicate in a living cell by using the energy and chemicals of their host cell [19,20,21,22,23]. Each type of virus can infect a limited range of host cells. A virus identifies a host cell. Then the virus matches and binds its outer proteins to specific receptor molecules on the surface of the cell. A viral infection starts when the genome of a virus enters a host cell. Consider the following simple viral replication cycle. A DNA virus, with a capsid containing a single type of protein, enters a host cell. After the virus enters the cell, the DNA is uncoated by enzymes to expose the genetic material. The viral DNA uses the host’s nucleotides and enzymes to replicate itself. It uses other host materials to produce its capsid proteins. It should be mentioned that RNA is synthesized from DNA (by the process of transcription) and specifies the structure of the capsid proteins. The DNA and capsid proteins assemble into new viruses. This cycle ends when the new viruses exit the cell. They are now able to infect additional cells and spread the infection.

Most DNA viruses use the DNA polymerase of the host cell to synthesize new genomes for the templates made available by the viral DNA [24]. On the other hand, RNA viruses replicate their genomes by using special virus-encoded polymerases that use RNA as a template. In any case, the host provides the nucleotides for the nucleic acid synthesis and the enzymes, etc. for making viral proteins dictated by messenger RNA, mRNA (that is transcribed from viral genes). The RNA viruses with the most complicated replication cycles are the retroviruses [25]. Retro (backward) refers to a reverse direction of flow for the genetic information of these viruses. A retrovirus inserts a copy of its RNA into the DNA of a host cell. The virus uses its own reverse transcriptase enzyme to produce DNA from its RNA genome. Then the host cell uses the viral DNA as part of its own genome to produce the required proteins to assemble new copies of the virus. An example of a retrovirus is HIV (human immunodeficiency virus) [26,27,28,29,30,31]. This virus targets the immune system. It attacks CD4 cells (immune cells, types of T cell). They are white blood cells that circulate and detect infections within the body. HIV uses CD4 cells to create more copies of the virus while decreasing the body’s ability to combat diseases, etc. An advanced stage of HIV infection is AIDS (acquired immunodeficiency syndrome). Treatments for HIV are called antiretroviral drugs. They can stop the progression of the infection and increase a person’s life expectancy. For protection against HIV, one should avoid the sharing of needles and exposure to body fluids. Figure 1.3 provides a diagram for the replication cycle of HIV [32].

Fig. 1.3
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This figure shows the replication cycle of HIV, a retrovirus

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2 Examples of Viral Infections

As previously mentioned, a virus is an infectious agent that is submicroscopic and replicates inside the living cells of an organism. It can also mutate. A mutation is a change in a virus’s genome, a set of genetic instructions needed for the virus to function [33,34,35,36,37,38]. It happens when a virus has contact with a host and starts to replicate. When the virus replicates, there is a chance that its genetic code is not copied correctly. The errors result in variants of the virus. Some mutations die out, while others survive and spread. Public health organizations are concerned about variants that increase infectivity or allow a virus to escape the immune system. Viruses can change genetically by a method called antigenic shift. For example, a nucleotide base is changed, inserted, or deleted from a DNA or RNA sequence of an organism’s genome. Figure 1.4 displays an antigenic shift resulting in a new and highly pathogenic strain of the human flu [39].

Fig. 1.4
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This figure shows an antigenic shift that results in a new and highly pathogenic strain of the human flu

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Mutations can be detected through genome sequencing, which reveals the order of bases (adenine, cytosine, guanine, and thymine) present in the entire genome of an organism. This technique (which involves computer software, etc.) allows scientists to monitor small changes in the virus to understand how it works. A person with a particular virus provides a swab sample. Then the genetic code can be extracted before being read using a sequencer. Finally, the genomes and mutations are compared. Then the information is shared with scientists.

The genetic material inside a virus helps determine how fast a virus mutates. This impacts how the illness spreads in the population. Viruses that replicate through DNA use the same process that the host cell uses to create its DNA. Therefore, mutations occur slowly. Smallpox is an example of such a DNA virus. Smallpox (now considered eradicated by the WHO) was an infectious disease caused by the Variola virus. Variola is a large brick-shaped virus with a single linear double-stranded DNA genome. The two virus variants (for smallpox) include Variola major and Variola minor [40, 41].

Initial symptoms for smallpox included fever and vomiting. These symptoms were followed by ulcers forming in the mouth and a skin rash resulting in fluid-filled blisters with a dent in the center. These blisters eventually scabbed over and fell off. However, scars remained. The disease was spread by infected people (especially through sneezes, coughs, etc.) and by contaminated items and surfaces. Smallpox vaccine was used to prevent this disease, which is no longer a problem. Dr. Edward Jenner, an English physician and scientist, created the world’s first smallpox vaccine.

RNA viruses, on the other hand, replicate using a more complex mechanism. Therefore, errors in genetic coding can occur in RNA viruses, like the influenza virus. These errors allow RNA viruses to quickly mutate from host cell to host cell. This makes it hard to keep up and prepare vaccines for new strains of the virus. A virus can be spread by insects, animals, and humans (especially through droplets from sneezes and coughs) [42]. The diagram of body infection sites (for viruses) is displayed in Fig. 1.5 [43]. Keep in mind that vaccines and antiviral drugs are used to help prevent and control the spread of viruses. The next section introduces and describes two types of viral infections: shingles and rabies.

Fig. 1.5
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This diagram shows infection sites in the human body

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Fig. 1.6
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This is an electron micrograph of varicella zoster virus magnified about 150,000×

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2.1 Shingles

Shingles is a viral infection that causes a painful rash with blisters [44,45,46,47]. The rash is generally in a localized area on the left or right side of the body. It usually heals within a few weeks, but some people develop nerve pain that can last for a long time. If the virus involves the eyes, then a loss of vision may occur. Shingles (herpes zoster) is caused by reactivation of the varicella zoster virus (a double-stranded DNA virus) which is responsible for chickenpox. See Fig. 1.6 [48]. After chickenpox infection, the virus remains dormant in the body’s nerve tissues. However, later in life, the virus can be reactivated and cause shingles. Once reactivated, the virus travels from the nerve body to the skin where it produces blisters. Figure 1.7 shows the progression of shingles starting with the dormant virus and ending with a rash, etc. on the surface of the skin [49]. Number 1 in the diagram represents bumps. Number 2 shows the blisters which break open (number 3) and crust over (number 4). Then they disappear. These steps take about 4 weeks. A painful condition called postherpetic neuralgia occurs sometimes. It is thought to be caused by nerve damage (number 5) and can last weeks or much longer after the rash disappears. Shingles is more likely to occur in older people and those who have impaired immune systems. NOTE: It is rare for a person to have shingles more than three times. Antiviral drugs may reduce the amount of time one suffers with the shingles and its severity. However, they need to be started within 72 h after the rash appears. The standard treatment has been acyclovir. The newer and superior antiviral drugs include valaciclovir and famciclovir. Vaccines are available to prevent shingles. Zostavax was licensed by the FDA in 2006. It is given as one shot and can reduce the risk of developing shingles by about 50%. This vaccine contains a weakened chickenpox virus. Shingrix requires two shots and is recommended for individuals aged 50 years and older. It is about 90% effective and does not contain live virus [50].

Fig. 1.7
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This diagram shows the progression of shingles (from dormant virus to the skin surface)

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2.2 Rabies

The rabies virus is a neurotropic virus, one that infects nerve cells [51,52,53,54,55]. It has the scientific name Rabies lyssavirus. This virus infects the central nervous system and causes inflammation of the brain in mammals and humans. It is mainly spread by bites and scratches from infected animals, which have the virus stored in their salivary glands. Also, transplants from donors who have died from rabies or unspecified encephalitis have transmitted rabies to humans. Most cases of rabies come from dogs, especially in developing and underdeveloped countries. According to the World Health Organization (WHO), India experienced a high death rate for humans (due to rabies) in 2004 [56]. Common sources of rabies in the United States include bats, raccoons, foxes, and skunks. Silver-haired bats and tricolored bats (formerly known as the eastern pipistrelle) are generally reported for causing cases of human rabies in the United States [57]. It should be mentioned that many tricolored bats previously died from the fungal disease known as the white-nose syndrome [58, 59]. Figure 1.8 shows the tricolored bat [60].

Fig. 1.8
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A photo of the tricolored bat is provided

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Initial symptoms for the rabies virus include a fever, a headache, and tingling at the site of exposure. As the disease progresses, a person might experience partial paralysis, confusion, abnormal behavior, a fear of water, and a coma. A sick individual usually dies 2–10 days after the onset of initial symptoms. The incubation period of this virus for humans is generally from 1–3 months. However, it may be as short as a few days or longer than a year.

The Rabies lyssavirus displays a cylindrical shape. It has a single-stranded RNA genome with negative sense. (A negative sense RNA virus consists of viral RNA that is complementary to the viral mRNA. On the other hand, positive sense RNA virus consists of viral mRNA that can be directly translated into proteins.) The rabies virus has a lipoprotein envelope with spikes composed of glycoprotein (G). Beneath the envelope is the membrane or matrix (M) protein layer. The core of the virus particle contains helically arranged ribonucleoprotein. Figure 1.9 displays a 3D structure of the rabies virus [61].

Fig. 1.9
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This figure shows the structure of a rabies virus

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Rabies can be prevented. Since dogs are the main source of human deaths from rabies worldwide, they should get pre-exposure vaccinations to prevent rabies in people. Also, oral vaccinations in pellet form need to be distributed and made available to wildlife. Individuals with high-risk occupations should receive pre-exposure immunizations too. These people include veterinarians, laboratory workers who handle rabies viruses, animal disease control staff, wildlife rangers, and more.

Bite victims need immediate treatment after being exposed to rabies. This fast action helps prevent the virus from entering the central nervous system, which usually results in death. First, the wound should be thoroughly washed and flushed (for at least 15 min) using soap and water, detergents, etc. to remove and kill the rabies virus. Postexposure prophylaxis involves one dose of rabies immune globulin and five doses of rabies vaccine within a 28-day period. The rabies immune globulin has antibodies from blood donors, who were given the rabies vaccine. The rabies vaccine stimulates a person’s immune system to make antibodies that neutralize the virus.

Animals can be diagnosed for rabies. Those displaying unusual behavior and other symptoms of the disease may have the virus. Postmortem diagnosis of rabies in animals is easier. Tests are carried out on parts of the affected brain. For example, one looks for virus antigens in brain tissue by using a fluorescent antibody technique. Other available methods include isolating the virus in a cell culture, etc.

3 Conclusions

This chapter includes background information about viruses. It describes a virus as an infectious agent that is submicroscopic and replicates inside the living cells of an organism. These organisms include plants, animals, and humans. Viruses contain a nucleic acid (DNA or RNA) and proteins. They exist in a variety of forms such as spheres, cubes, rods, etc. Some (like the influenza virus) are surrounded by a membrane envelope that is covered with spikes that project outward from the surface. The replication process of viruses, including the retrovirus HIV (human immunodeficiency virus), is presented and described. (Retro, backward, refers to a reverse direction of flow for the genetic information of these viruses.) Viruses can mutate during the replication process. A mutation is a change in a virus’s genome, a set of genetic instructions needed for the virus to function. It happens when a virus has contact with a host and starts to replicate. When the virus replicates, there is a chance that its genetic code is not copied correctly. Errors result in variants of the virus. Some mutations die out, while others survive and spread. Public health organizations are concerned about variants that increase infectivity or allow a virus to escape the immune system.

In addition, two types of viral infections (shingles and rabies) are introduced and described. Information is provided about their virus structure, cause of disease, transmission, symptoms, methods of prevention and treatment, etc.