Human herpes viruses (HHVs) refer to a group of eight DNA viruses that affect humans, known as HHV-1, HHV-2, HHV-3, HHV-4, HHV-5, HHV-6, HHV-7, and HHV-8. Although a considerable proportion of the human population is infected with one or more of these viruses, the social awareness and understanding of them are not enough, much less than that of some other equally prevalent pathogens. To protect yourself from these viruses, it's essential to acquire some knowledge of them.

The eight viruses can be divided into three families:

HSV-1, HSV-2, and HHV-3 belong to this family. Their major characteristics are an extremely short reproductive cycle (about hours), rapid destruction of host cells, and the capacity to replicate in various host tissues. They are able to develop latent infection in sensory nerve ganglia.

HHV-5, HHV-6, and HHV-7 belong to this family. They have a long reproductive life (about days), a restricted host range, and the ability to form enlarged cells. They are able to develop latent infection in secretory glands, cells of thereticuloendothelial system, and the kidneys.

HHV-4 and HHV-8 belong to this family. They have the most restricted host range and replicate in lymphoblastoid cells in vitro. Latency of these viruses is often demonstrated in lymphoid tissue.

All herpesvirus virions have a unique, complex structure that consists of four components:

1. A core that contains a single linear molecule of double-stranded DNA (dsDNA)

2. A capsid constructed of capsomers, which surrounds the core

3. A tegument, which is an amorphous protein coat between the capsid and envelope

4. An envelope, which is the outer layer of the virion and is a glycoprotein-bearing lipid bilayer membrane

These viruses cause both latent or lytic infections. They can lie dormant, or latent, in the body for years and reactivate to cause recurrent disease under certain conditions. An important strategy herpesviruses use to establish successful infection is to hide their genome in the nucleus of host cells. In most cases, latent herpesvirus infections are benign. But for newborns and immunosuppressed persons, reactivation of latent herpesvirus infections can be fatal.

HHV-1, or called herpes simplex virus 1 (HSV-1), often causes oral herpes (painful sores or blisters in or around the mouth) but sometimes also causes genital herpes. Oftentimes, HSV-1 infection causes no symptoms. That's why most infected people do not realize that they are infected. But for individuals with weakened immune systems such as newborns, the elderly and immunocompromised persons, HSV-1 infection can lead to frequent recurrences and severe complications such as neonatal herpes, encephalitis (brain inflammation), and keratitis (eye inflammation).

HSV-1 is epidemic, affecting 67% of people under the age of 50. It is highly contagious and is often transmitted by oral-to-oral contact. Oral-genital contact, such as during oral sex, also spreads the virus. Although rare, a mother with genital HSV-1 infection may spread the virus to her baby during delivery. The virus is more likely to be transmitted through sharing personal items such as eating utensils, razors, and towels from an infected person with active HSV-1 lesion.

HHV-2, or called herpes simplex virus 2 (HSV-2), often causes genital herpes. Like HSV-1, HSV-2 infection often goes unrecognized. HSV-2 infection can cause severe complications such as meningoencephalitis (inflammation of the meninges and brain), esophagitis (esophagus inflammation), hepatitis (liver inflammation), pneumonitis (lung inflammation), and retinal necrosis in HIV-infected patients and other immunocompromised persons.

HSV-2 is also spread by skin-to-skin contact, particularly sexual contact. Mother-to-child transmission during delivery is rare. Although less common than HSV-1, HSV-2 is also widespread, affecting more than 400 million people worldwide. Women are more susceptible to HSV-2 infection than man.

At present, there are no effective vaccines to prevent HSV-1 or HSV-2 infection.

HHV-3 is also known as varicella-zoster virus (VZV). Primary VZV infection causes chickenpox, often seen in children, teens, and young adults, while recurrent VZV infection causes shingles, often seen in older adults. Chickenpox is a fairly mild, self-limited illness that generally goes away within a few days, but in some cases, it can lead to hospitalization and even death due to severe complications such as inflammation of the brain, lung or other tissues, bleeding problems, sepsis, and dehydration. Shingles (herpes zoster) is a painful, maculopapular rash usually on one side of the body.

VZV is easily spread via directly touching the blisters, saliva or mucus of an infected person, via breathing in the air when an infected person is coughing or sneezing, or via touching contaminated items.

There are already vaccines available to prevent VZV infection. Getting vaccinated is an important strategy to prevent yourself from getting the disease.

HHV-4, or called Epstein-Barr virus (EBV), is the major cause of infectious mononucleosis. In addition, EBV is linked to other diseases, including autoimmune diseases, neurodegenerative diseases and certain types of cancer.

EBV is transmitted mainly through bodily fluids, especially saliva, and also through blood and semen during sexual contact, blood transfusion, and organ transplantation. An uninfected person may get infected by sharing personal items such as a toothbrush or drinking glass from an infected person. The virus may survive on an object if the object remains moist. This is one of the reasons why EBV is so prevalent.

A vaccine against EBV is not yet available.

HHV-5 is the official name of cytomegalovirus (CMV). CMV is also a cause of mononucleosis. Other diseases associated with CMV include Guillain-Barré syndrome, type 1 diabetes, and type 2 diabetes. Some of the possible transmission modes of CMV are sexual contact, breast feeding, blood transfusions, and organ transplants. CMV is present in various body fluids, such as urine, saliva, blood, tears, semen, and breast milk. A person may get infected by close contact through sex and through saliva, urine, and other body fluids. 

CMV infects 50-80% of people at some time during their lives but rarely causes obvious illness. If illness occurs, symptoms may include diarrhea, severe vision problems including blindness, infections of the stomach and intestines, and even death. In patients with AIDS, CMV infection is hard to treat.

Unfortunately, no cure or vaccine is available to prevent CMV infection in humans. A recent research suggests a cellular protein called EB3 as a druggable target to suppress CMV replication.

HHV-6B is the causative agent of roseola and is linked to fatal neurological diseases like encephalitis, and organ transplant failure. HHV-6B is commonly found in the saliva after primary infection and is spread to infants from family members and caregivers. HHV-6B affects nearly 100% of the human population, making it one of the most successful pathogens existing.

Relatively little is known regarding HHV-6A, but there is evidence that HHV-6A is more neurovirulent. HHV-6A is more frequently found in patients with neuroinflammatory diseases such as multiple sclerosis, and appears to be a factor in female infertility. HHV-6A is generally not found in the saliva, and is encountered much less frequently in North America, Europe, and Japan.

HHV-7 is closely related to HHV-6. Like HHV6, HHV-7 infects nearly all people. Infection with HHV-7 usually starts early in life (before the age 3). HHV-7 can also cause roseola, but it's believed that there are other conditions caused by this virus. Contact with saliva of an infected person may be the major transmission mode of HHV-7.

HHV-8, or Kaposi's sarcoma-associated herpesvirus (KSHV), is the underlying infectious cause of Kaposi sarcoma, KSHV associated multicentric Castleman's disease, primary effusion lymphoma, and KSHV inflammatory cytokine syndrome. The prevalence of HHV-8 varies greatly among countries and regions. The prevalence of HHV-8 infection in sub-Saharan Africa is among the highest in the world; About 90% of KSHV occurs in sub-Saharan Africa where it is endemic and primarily transmitted via non-sexual contacts. HHV-8 infection rates are about 19.2% in Xinjiang compared to about 9.5% in Hubei, China. Oral-genital sexual contact appears to be the most risky act for the transmission of HHV-8. But there are other transmission modes.

In summary, these eight herpes viruses are very common among mankind. These viruses can remain latent and reactivate to cause illness later. Many of the viruses do not cause disease or symptoms in the majority of cases. But for people with weakened immune systems, these viruses are more likely to cause severe complications. Our understanding of some of the eight viruses is very limited, hindering the development of therapeutics and vaccines.

Caroline Liu is a writer and contributes content on topics such as common viruses, diseases, lifestyles, health, and biotech. She works at Cusabio, a supplier of proteins, antibodies, and Elisa kits that serves researchers in the field of life sciences research.