A viral infection occurs when tiny disease-causing particles, called a virus, enter the body and begin multiplying. More than 400 different viruses are known to cause infections in humans.
Once a virus enters the body, it uses the host's cells to reproduce. In some cases, the infected cell is destroyed during this process. Once the virus multiplies, new viral particles are released into the body that can infect more cells. As the virus takes control over certain cells, the person starts to become sick. Symptoms vary depending on the specific type and number of cells that become infected.
Some viruses, such as the human papilloma virus (HPV), do not kill the cells they infect. Instead, some viruses may just change the host cell's functions. For instance, some cells may begin to multiply and divide abnormally, which may lead to cancerous growths. Other viruses may insert their genetic material (DNA) into a human cell where it remains dormant (latent) for a period of time. When the cell is disturbed, it may stimulate the virus to start multiplying, which causes an illness.
Most viruses infect specific types of cells in the body. For instance, the human immunodeficiency virus (HIV) primarily attacks immune cells called CD4 cells. Because the CD4 cells fight against disease and infection, the virus weakens the patient's immune system.
Viral infections can affect most parts of the body, including the liver, immune system, and skin. The severity of a viral infection depends on the specific type of infection. Some viruses, such as the common cold, may cause mild symptoms that go away on their own in a few days. Other viral infections, such as hepatitis, may be life threatening.
Mild viral infections usually do not require treatment. Some patients with the common cold or flu may benefit from medications, such as nasal decongestants, to treat the symptoms. More severe viral infections, such as hepatitis, may require medications such as antivirals or immunomodulators to destroy the virus. Not all viral infections are curable. For instance, HIV is a lifelong condition that eventually leads to death. However, HIV treatment, called antiretroviral therapy, delays the progression of the disease by limiting the virus's ability to multiply in the body. As a result, less HIV is present in the blood.
Many types of viral infections can be prevented with vaccinations, also called immunizations. Patients can receive vaccinations to prevent hepatitis A, hepatitis B, influenza, as well as the measles, mumps, and rubella (MMR).
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viral life cycle
General: Although it has been debated whether or not viruses are living organisms, most experts considered them non-living. This is because viruses do not meet all of the criteria to be classified as living organisms. For instance, viruses do not respond to stimuli, which is a common characteristic of living things.
The life cycle of a virus can be simplified and divided into five stages: attachment, penetration, uncoating, replication, and release. Uncoating only occurs in viruses that have outer shells.
Attachment: The virus attaches to specific proteins, called receptors, on the outer surface of human cells.
Penetration: Following attachment of the virus to membrane surface proteins, it enters the cell.
Uncoating: If the virus has an outer shell, it will go through a stage called uncoating. During this process, the virus releases its outer shell and expels its contents to prepare for replication.
Replication: Viral proteins and genetic material are assembled to make multiple copies while still inside the host cell.
Release: After many copies of the virus have been made, they are released from the host cell, often killing the host cell in the process. These newly formed viruses are now able to infect additional cells.
common types of viral infections
Common cold: The common cold, also called acute viral nasopharyngitis, is a viral infection of the upper respiratory system, which may involve the nose, throat, sinuses, eustachian tubes (connects the ears to the throat), windpipe (trachea), voice box (larynx), and/or airways (bronchial tubes).
Common colds generally cause mild symptoms, such as runny nose and sore throat, which resolve on their own in about a week.
Patients can develop an infection after direct contact with the cold virus. This may happen when an individual touches environmental surfaces that have cold germs on them (e.g. public telephones, computer keyboards, or stair railings) and then touches the eyes, nose, or mouth. Colds may also be transmitted if a person inhales droplets of mucus that are contaminated with the cold virus. Mucus droplets may enter the air when an infected person coughs or sneezes. These droplets are invisible to the human eye and can remain in the air for several minutes.
Hepatitis A: Hepatitis A is a highly contagious liver infection that is caused by the hepatitis A virus (HAV). Although not usually as serious as other types of viral hepatitis, hepatitis A causes inflammation of the liver that may affect the liver's ability to function. The liver is primarily responsible for filtering most of the nutrients that are absorbed in the intestines, as well as removing drugs, alcohol, and toxins from the bloodstream.
Hepatitis A is transmitted when contaminated feces enter the mouth. This usually happens when a person consumes foods (especially produce that grows in the soil or food that is prepared by someone with dirty hands) or water that is contaminated.
Most cases of hepatitis A cause mild, if any, symptoms and do not require medical treatment. However, in rare cases, hepatitis A can be life threatening and cause liver failure.
Hepatitis B (HBV): The hepatitis B virus causes a serious liver infection. The infection can become chronic in some people and lead to liver failure, liver cancer, cirrhosis (permanent scarring and damage of the liver), or death.
The hepatitis B virus is transmitted through contact with bodily fluids such as the blood, semen, and vaginal discharge of someone who is infected. Even though HBV is transmitted by the same methods as HIV, hepatitis B is nearly 100 times more infectious. This is because HBV is more concentrated than HIV. Individuals of any age, race, nationality, gender, or sexual orientation can become infected with HBV.
There are few treatment options for patients with chronic hepatitis B, which is why people in the United States are required to receive the hepatitis B vaccination to prevent infection. In some cases, the doctor may suggest monitoring the patient's condition instead of treating it. In other instances, the doctor may recommend antiviral treatment or immunomodulatory therapy. When liver damage is severe, a liver transplantation may be the only treatment option.
Human immunodeficiency virus (HIV): The human immunodeficiency virus (HIV) is a virus that causes AIDS (acquired immune deficiency syndrome). HIV destroys important immune cells that are needed to fight off disease and infection. As a result, HIV patients are more likely to become sick.
HIV is transmitted from person to person via bodily fluids, including blood, semen, vaginal secretions, and breast milk. Therefore, it can be transmitted through sexual contact with an infected person, sharing needles/syringes with someone who is infected, breastfeeding, vaginal birth or, less commonly (and rare in countries where blood is screened for HIV antibodies), through transfusions with infected blood.
HIV is a fatal condition. Although antiretrovirals can help suppress the virus, they do not completely eliminate it from the blood. Once patients develop AIDS, they have an increased risk of developing fatal infections, such as pneumonia.
Influenza (flu): Influenza, commonly referred to as the flu, is a contagious viral infection of the respiratory system.
In general, the flu is more debilitating than the common cold and symptoms, such as fever, body ache, extreme fatigue, and dry cough, are more common and severe in patients with the flu. Infants, the elderly, and chronically ill patients have the greatest risk of developing severe cases of the flu that may be life threatening.
The flu is caused by a slightly different type of virus than the common cold. Patients can develop an infection after direct contact with the flu virus. This may happen when an individual touches environmental surfaces that have cold germs on them (e.g. public telephones, computer keyboards, or stair railings) and then touches the eyes, nose, or mouth.
Mononucleosis (mono): Mononucleosis, also called mono or the kissing disease, is a viral infection of white blood cells called monocytes.
Mono is caused by the Epstein-Barr virus. Although the virus is less contagious than the common cold, it may still be passed on to others. It is transmitted through saliva and airborne mucus droplets. Individuals can acquire the disease after kissing an infected person, sharing food utensils, or inhaling mucus droplets.
The infection resolves on its own. Patients do not receive antivirals because these medications may actually worsen the condition. Patients may receive treatment to help reduce symptoms. Most symptoms begin to improve after a few weeks. However, fatigue and an enlarged spleen and lymph nodes may take a few weeks longer to improve.
Measles, mumps, and rubella: Measles, mumps, and rubella are highly contagious viral infections. However, they are rare in countries, such as the United States, where individuals have access to vaccinations.
Measles is a viral infection of the respiratory tract. Mumps is a viral infection of the salivary glands that causes swelling. Rubella, also called German measles, is a mild infection of the respiratory tract that often goes unnoticed. However, if a pregnant woman develops rubella, it may lead to birth defects in the infant.
These infections are transmitted through airborne droplets. People become infected with the viruses when they inhale particles of infected sputum from the air. The viruses become airborne when an infected person expels saliva when they cough, sneeze, talk, spit, etc.
Molluscum contagiosum: Molluscum contagiosum is a viral infection of the skin that primarily affects children.
In children, small bumps are most likely to develop on the neck, arms, and face. In adults, small bumps usually develop on the genitals, upper thighs, and lower abdomen. When the genital region is affected in adults, it is considered a sexually transmitted disease (STD).
Molluscum contagiosum can spread through direct contact with infected skin. Individuals can also develop the infection after touching objects (e.g. towels, doorknobs, bed linens, or clothing) that came into contact with an infected person's skin.
Patients should not scratch or rub affected areas of the skin because it may cause the virus to spread to others areas. Heat and moisture may cause the virus to multiply faster.
Molluscum contagiosum does not cause any serious health problems. If left untreated, molluscum contagiosum will resolve on its own within one year. However, because the infection is highly contagious, treatment is recommended.