Herpesviruses are a widespread family of DNA viruses (Herpesviridae), with eight distinct types commonly infecting humans. These viruses are characterized by their complex structure. They are highly prevalent globally, with many individuals carrying at least one type. A defining feature is their ability to establish lifelong infections, meaning the virus remains permanently in the body once infected.
Understanding Herpesviruses
A fundamental characteristic of all herpesviruses is their capacity for latency, where the virus remains dormant within host cells after initial infection. During this latent phase, the viral genetic material persists with minimal or no active replication. This allows the virus to evade the host’s immune system.
However, latent herpesviruses can reactivate, leading to a recurrence of symptoms or asymptomatic shedding. Reactivation can be triggered by various factors, including stress, fever, hormonal changes, ultraviolet light exposure, or a weakened immune system due to illness or medication. When reactivation occurs, the virus begins to replicate, which can cause active disease. Even without noticeable symptoms, viral shedding during reactivation can lead to transmission to others.
Alphaherpesviruses
The Alphaherpesvirinae subfamily includes three human herpesviruses known for their rapid replication cycle and ability to establish latency primarily in nerve cells. These viruses can cause a variety of conditions, often involving skin and mucous membranes.
Human Herpesvirus 1 (HHV-1)
Human Herpesvirus 1 (HHV-1), or Herpes Simplex Virus 1 (HSV-1), primarily causes oral herpes, known as cold sores or fever blisters. It spreads through direct oral contact, such as kissing or sharing utensils. While HSV-1 primarily causes lesions around the mouth, it can also lead to genital herpes.
Human Herpesvirus 2 (HHV-2)
Human Herpesvirus 2 (HHV-2), or Herpes Simplex Virus 2 (HSV-2), is primarily associated with genital herpes, characterized by painful blisters and ulcers in the genital or anal area. HSV-2 is transmitted through sexual contact. Both HSV-1 and HSV-2 can cause severe complications in immunocompromised individuals and can be transmitted from a mother to a newborn during birth, potentially causing neonatal herpes.
Human Herpesvirus 3 (HHV-3)
Human Herpesvirus 3 (HHV-3), or Varicella-Zoster Virus (VZV), is responsible for two distinct diseases. Primary infection causes chickenpox, a highly contagious childhood illness marked by an itchy, blistering rash, spread through respiratory droplets or direct contact with fluid from blisters. After chickenpox resolves, VZV remains latent in nerve cells and can reactivate later in life as shingles (herpes zoster), a painful rash that often appears in a band or stripe on one side of the body.
Betaherpesviruses
The Betaherpesvirinae subfamily consists of human herpesviruses characterized by a slower replication cycle and the ability to infect a broader range of cell types, often causing enlarged cells. These viruses frequently establish latency in secretory glands, kidneys, and white blood cells.
Human Herpesvirus 5 (HHV-5)
Human Herpesvirus 5 (HHV-5), or Cytomegalovirus (CMV), is highly prevalent, with most adults having been exposed. In healthy individuals, CMV infection is often asymptomatic or causes mild, mononucleosis-like symptoms. However, CMV poses a significant risk to immunocompromised individuals, such as transplant recipients or those with HIV, leading to severe complications affecting organs like the lungs, liver, or eyes. It can also cause severe congenital defects if transmitted to a fetus during pregnancy. CMV spreads through close contact with infected bodily fluids, including saliva, urine, blood, and sexual contact.
Human Herpesvirus 6 (HHV-6)
Human Herpesvirus 6 (HHV-6) includes two distinct species: HHV-6A and HHV-6B. HHV-6B is highly common, infecting most children by age three, and is the primary cause of roseola infantum, a childhood illness characterized by a high fever followed by a rash. It is primarily transmitted through saliva. HHV-6A is less understood and less common in the general population, but has been linked to neuroinflammatory diseases and is more frequently found in immunocompromised individuals. Both variants can remain latent and reactivate, especially in those with weakened immune systems, potentially causing serious complications like encephalitis.
Human Herpesvirus 7 (HHV-7)
Human Herpesvirus 7 (HHV-7) is also widespread, with most people contracting it during childhood, often asymptomatically. Like HHV-6B, HHV-7 can sometimes cause roseola infantum. Its modes of transmission are similar to HHV-6, primarily through saliva. While HHV-7 generally causes mild or no symptoms in healthy individuals, its role in disease, particularly in immunocompromised settings, is still under investigation.
Gammaherpesviruses
The Gammaherpesvirinae subfamily primarily targets lymphocytes, a type of white blood cell, and can establish latency in lymphoid tissues. These viruses have been linked to various proliferative disorders and certain cancers.
Human Herpesvirus 4 (HHV-4)
Human Herpesvirus 4 (HHV-4), or Epstein-Barr Virus (EBV), is one of the most common human viruses, infecting over 90% of adults worldwide. EBV causes infectious mononucleosis, often called “mono” or the “kissing disease,” due to its transmission through saliva. Symptoms include extreme fatigue, fever, sore throat, and swollen lymph nodes. EBV has also been associated with an increased risk of certain cancers, including Burkitt lymphoma, Hodgkin lymphoma, and nasopharyngeal carcinoma.
Human Herpesvirus 8 (HHV-8)
Human Herpesvirus 8 (HHV-8), or Kaposi’s Sarcoma-Associated Herpesvirus (KSHV), is the most recently discovered human herpesvirus and is recognized as an oncovirus. KSHV causes Kaposi’s sarcoma, a type of cancer that affects the skin, lymph nodes, and internal organs, particularly common in individuals with weakened immune systems, such as those with HIV/AIDS. It is also linked to other lymphatic cancers like primary effusion lymphoma and multicentric Castleman disease. KSHV is primarily transmitted through saliva, but can also spread sexually or through blood and organ transplants.