Viral Infections

Smallpox is a severe contagious disease caused by the Variola major virus. This virus is a double-stranded DNA member of the Poxviridae family. It spreads mainly through inhaling virus-laden droplets or through direct contact with infectious scabs.

After exposure, the incubation period is usually about seven days. It can range from 7 to 17 days, depending on the amount of virus and host factors. The first symptoms appear suddenly and include high fever, malaise, headache, and myalgia, which...

Herpes simplex virus type 1, or HSV-1, is a widespread double-stranded DNA virus that causes orolabial lesions. It is an enveloped virus in the Herpesviridae family. After it enters a host cell, its DNA travels to the nucleus, where immediate-early, early, and late genes are turned on in order. This gene sequence supports viral DNA copying, structural protein production, and new virion assembly.

HSV-1 first infects epithelial cells, then it can become latent in sensory neurons of the...

Chickenpox is a highly contagious disease caused by the varicella-zoster virus (VZV). VZV is a double-stranded DNA virus in the Herpesviridae family. The infection spreads through respiratory droplets or by direct contact with fluid from skin blisters.

After exposure, the virus usually incubates for 10 to 21 days. During this time, it replicates and moves through the body in stages. Chickenpox is often mild and self-limiting in children, but it can cause serious complications in neonates,...

Viral meningitis affects the membranes around the brain and spinal cord and is the most common form of meningitis. It is also called aseptic meningitis because bacteria are not involved. In many cases, it is milder than bacterial meningitis, but it can still cause fever, headache, stiff neck, drowsiness, nausea, photophobia, and vomiting.

Enteroviruses are common causes of viral meningitis. These include coxsackie A and B viruses and echoviruses. They belong to the Enterovirus genus in the...

Poliovirus spread and prevention are key to understanding poliomyelitis. Poliomyelitis is caused by poliovirus, a small, non-enveloped, positive-sense RNA virus in the Picornaviridae family and Enterovirus genus. The virus spreads mainly by the fecal-oral route, often through contaminated water or food.

After infection, poliovirus first replicates in the oropharynx and intestinal mucosa. It often multiplies in lymphoid tissues such as the tonsils, Peyer’s patches, and regional lymph nodes. A...

Rabies is a lethal zoonotic disease caused by a single-stranded, negative-sense RNA virus from the Lyssavirus genus in the Rhabdoviridae family. It usually spreads to humans through bites or saliva-contaminated scratches from infected mammals such as dogs, bats, raccoons, or foxes. The virus can also spread when infectious saliva contacts broken skin or intact mucous membranes, including the conjunctiva.

After a bite or scratch, rabies first enters local muscle cells by receptor-mediated...

West Nile virus can cause brain inflammation when it reaches the central nervous system. This arboviral infection is spread by mosquitoes, especially in a cycle that involves birds as amplifying hosts and mosquitoes as vectors. Humans and other mammals are incidental dead-end hosts. Most human infections never become severe, but some progress to neuroinvasive disease.

West Nile virus, or WNV, is an enveloped, positive-sense, single-stranded RNA virus in the Flaviviridae family. Infection...

Yellow fever is a viral hemorrhagic disease that spreads through mosquito bites and can damage the liver. The yellow fever virus, or YFV, belongs to the Flaviviridae family. It is transmitted mainly by Aedes and Haemagogus mosquitoes in tropical and subtropical regions of Africa and South America.

After a mosquito bite, the virus first infects immune cells in the skin, including dendritic cells and macrophages. These cells move to the lymph nodes, where the virus multiplies further. The...

Cytomegalovirus (CMV) is a human double-stranded DNA virus in the Herpesviridae family. CMV disease can be mild or silent in many healthy people, but it can cause serious illness in newborns and in people with weakened immune systems. It is also a major cause of congenital viral infection in the United States and a common problem in solid organ and hematopoietic stem cell transplant recipients.

CMV spreads through bodily fluids, sexual contact, organ transplantation, blood transfusions, and...

Influenza is a highly contagious viral disease of the respiratory tract. It causes seasonal epidemics around the world. Influenza A is the most common type linked to large outbreaks. It is classified by two surface glycoproteins, hemagglutinin, or H, and neuraminidase, or N, as in H1N1.

These glycoproteins help the virus infect cells, spread between people, and avoid immune detection. Influenza spreads mainly through respiratory droplets and contaminated surfaces. People in high-risk groups...

Coronaviruses are enveloped viruses with a single-stranded, positive-sense RNA genome and a helical nucleocapsid. Their club-shaped spike glycoproteins stand out on the viral envelope and help the virus attach to host cells. Most coronaviruses infect the upper respiratory tract and often cause mild or no symptoms. Some strains, including SARS-CoV, can cause severe respiratory disease, especially in people at higher risk.

SARS-CoV starts infection when its spike protein binds to the...

Respiratory syncytial virus (RSV) is a common respiratory virus that can infect the lungs and airways. It mainly causes illness in infants and young children, but it can also seriously affect older adults and people with weak immune systems. RSV belongs to the Pneumoviridae family and is a negative-sense, single-stranded RNA virus in the Pneumovirus genus.

RSV has a major global health impact. It causes millions of infections each year and leads to hospitalizations and deaths, especially in...

Hepatitis is inflammation of the liver. It is most often caused by hepatotropic viruses, which are viruses that infect the liver. Alcohol and some drugs can also cause it.

Hepatitis A virus, or HAV, is a non-enveloped RNA virus in the Picornaviridae family. It usually spreads through the fecal-oral route, often from contaminated food or water. After it is swallowed, HAV can enter the bloodstream through the oropharynx or intestinal epithelium and then reach the liver.

HAV infects hepatocytes,...

Genital herpes is a sexually transmitted infection caused mainly by herpes simplex virus type 2 (HSV-2). Herpes simplex virus type 1 (HSV-1) is also increasingly linked to genital infections, especially in younger people. The virus spreads mainly through sexual contact. A major challenge is asymptomatic viral shedding, which means people can pass the virus on even when they have no symptoms.

HSV-2 is common worldwide. The World Health Organization estimates that about 13% of people ages 15 to...