Types of Human Viruses

Viruses are the smallest organisms that are known as obligate intracellular parasites. They are so small that they can invade and infect even the microscopic bacterial cells. These viruses invading bacteria are called bacteriophages. It is thought there are no living cells in the world that cannot be infected with one or more types of viruses.

There are different types of viruses that invade the plant cells, animal cells as well as human cells. Here, we shall learn more about the different types of human viruses that invade the human body and cause diseases.

When a virus is present outside a living cell, it exists in form of a virion. These virions are as small as 30 nm in diameter or as large as 230 nm. This virion is made up of an outer shell, called the protein capsid. The capsid is very important in protecting the core and deciding which cell the virion should be able to attach.

The core consists of the viral genome. This genome can either be a DNA or RNA molecule. After the virion attaches itself to the host cell, the DNA or RNA starts the synthesis of proteins that help in replicating the genome as well as synthesis of new proteins that will form the core as well as capsid.

As we have seen above, viruses either have a DNA or RNA as their genetic material. Thus, viruses are divided based on their genetic material, that is, DNA viruses and RNA viruses.

The DNA viruses are further divided into double stranded DNA molecule virus (dsDNA). For examples, smallpox virus is a dsDNA virus. Others that have a single stranded DNA molecule are called single stranded DNA molecule virus (ssDNA). The Adeno-Associated Virus (AAV) is a type of ssDNA virus.

The RNA viruses are also divided into further groups like single stranded antisense RNA virus, also called the negative stranded RNA viruses. These include the Ebola viruses. The second group of RNA viruses include those with a single stranded sense RNA are also called positive stranded RNA.

For example, polio virus. Those viruses with many pieces of double stranded RNA viruses like reovirus make up the third group of RNA viruses. And lastly, the retroviruses are those RNA viruses that have a single stranded RNA molecule that is copied by reverse transcriptase into a DNA genome once inside a host cell.

For example, the dreaded HIV. Based on these groups of viruses, let us have a look at the list of human viruses.

Here are DNA viruses that invade human cells:

Double Stranded DNA Viruses

• Adenoviruses: These viruses generally cause respiratory illnesses, but can also lead to conjunctivitis, cystitis, gastroenteritis and rash.
• Smallpox: The smallpox virus lead to a disease called variola vera that lead to eruption of a maculopapular rash and fluid filled blisters.

• Vaccinia: This virus was used for immunization against smallpox until the disease was eliminated completely.
• Varicella-zoster: This virus causes chickenpox when invades the body for the first time. When reactivated, it leads to a shingles infection.
• Hepatitis B: Causes hepatitis

• Herpes viruses: This includes the group of following viruses:
  * HSV-1: Causes cold sores on the lips and mouth.
  * HSV-2: Causes genital herpes
  * KSHV: Also called Human herpes virus 8 (HHV-8), it causes Kaposi's sarcoma in AIDS patients and those with suppressed immune systems.

* Human cytomegalovirus (HCMV): Infects most of the population, but can lead to blindness and even death in people with suppressed immune systems.
* Epstein-Barr virus (EBV): causes mononucleosis and may lead to Burkitt's lymphoma and Hodgkin's disease.

Here are RNA viruses that can lead to infections in humans:

Negative Stranded RNA Viruses

• Ebolavirus (EBOV): This virus leads to viral hemorrhagic fever.
• Influenza virus: This virus leads to an infectious disease called flu that leads to severe chills, fever, sore throat, headache and general discomfort.
• Morbillivirus: This virus leads to infection of the respiratory system called measles.
• Mumps virus

• Rabies virus: This virus leads to a fatal disease in animals as well as humans.
• Respiratory syncytial virus (RSV): Infection by this virus leads to lower respiratory tract infection.
• Parainfluenza viruses (PIV): Leads to lower respiratory tract infection.
• Human metapneumovirus: Causes an infection similar to RSV, but is found to be less severe infection.

Positive Stranded RNA viruses

• Coronaviruses: These include the virus that causes Severe Acute Respiratory Syndrome (SARS)
• Dengue fever viruses: Transmitted through mosquito bites and can lead to life-threatening dengue hemorrhagic fever.
• Hepatitis A and hepatitis C viruses: These viruses lead to acute infectious disease of the liver.

• Noroviruses: The virus that causes gastrointestinal infections especially in people living in close quarters like ships.
• Rhinoviruses: This group includes 99 strains of virus that can lead to common cold infection.

• Rubella viruses: This virus causes German measles.
• West Nile virus
• Yellow fever virus: This is also a mosquito transmitted virus that leads to an acute viral hemorrhagic disease.

Retroviruses

• HIV-1 and HIV-2 viruses : These are human immunodeficiency viruses that leads HIV/AIDS in humans.
• HTLV-1 and HTLV-2: The human T-cell lymphotropic virus type 1 (HTLV-1) infection leads to leukemia in about 3% patients.

These are the different types of human viruses that invade human cells and lead to infections.

Some other types of viruses include viruses that cause a lytic infection like the Epstein-Barr virus (EBV) that leads to mononucleosis. The latent infections of Epstein-Barr virus puts the patient at risk of lymphoma. Hope you have learned a bit about the viruses that can cause serious as well as fatal infections in humans.