Viral infections are common and very contagious. They induce a wide variety of manifestations. Examples of viral infections are nasopharyngitis, most tonsillitis and the flu.

A virosis is an infection caused by a virus. Viruses are ultra-microscopic organisms made up of genetic material (RNA or DNA nucleic acid) surrounded by a capsid made up of proteins and sometimes an envelope. They cannot feed and multiply on their own by division (whereas bacteria are microscopic single-celled living organisms that can feed and multiply).

Viruses need a host cell to survive and develop. Pathogenic viruses are viruses capable of causing disease with symptoms.

Viruses cannot infect all types of cells. Each virus has a more or less broad specificity which one defines as tropism. There are viruses with respiratory, digestive, genital, hepatic and neurological tropism. However, some viruses have multiple tropisms.

Examples of target organs for different viruses:

• Central nervous system: herpes simplex virus (HSV), cytomegalovirus (CMV), enterovirus, measles, mumps, rabies, arbovirus;
• Eye: measles, rubella, HSV, varicella zoster virus (VZV), CMV;
• Oropharynx and upper airways: rhinovirus, influenza, adenovirus, coronavirus, parainfluenza virus, HSV, CMV;
• Lower respiratory tract: influenza, measles, adenovirus, CMV;
• Gastrointestinal tract: enterovirus, adenovirus, rotavirus; 
• Liver: hepatitis A, B, C, D and E virus;
• Genitals: papillomavirus, HSV;
• Bladder: adenovirus 11;
• Peau: VZV, poxvirus, papillomavirus, HSV.

Acute viral infections (the most common) heal within a few days and up to a few weeks. Some viruses, such as hepatitis B virus and hepatitis C virus, can persist as chronic infections (continuous detection of the virus). Viruses of the Herpesviridae family (HSV, VZV, CMV, EBV) persist in a lifelong latent form in the organism (absence of detectable viral multiplication) and can therefore reactivate (new production of viral particles) in a large situation. fatigue, stress or immunosuppression (organ transplants, HIV infection or cancer).

Bronchiolitis

In France, each year, 500 infants (ie 000% of the infant population) are affected by bronchiolitis. Bronchiolitis is an epidemic viral infection that occurs mainly in children under two years of age.

It corresponds to an inflammation of the bronchioles, the smallest respiratory ducts of the lungs. Their obstruction is accompanied by very characteristic wheezing that occurs during breathing called wheezing. Bronchiolitis occurs mainly from October to April. It lasts about a week, the cough may persist a little longer. In more than 70% of cases, the virus responsible is RSV, Respiratory Syncytial Virus.

He is very contagious. It is spread from infant to infant or adult to infant through hands, saliva, coughs, sneezes and contaminated objects. RSV infection presents two complication risks: the acute risk of developing a severe form of the disease requiring hospitalization and the longer-term risk of developing “post-viral bronchial hyperresponsiveness”. This is manifested by repeated episodes with wheezing during breathing.

Influenza

Influenza is a viral infection caused by the Influenza virus, which includes three types: A, B and C. Only types A and B can give severe clinical forms.

Seasonal influenza occurs in the form of epidemics in mainland France. 2 to 6 million people are affected by the flu each year. The seasonal flu epidemic usually occurs between the months of November and April. It lasts an average of 9 weeks.

Influenza can cause severe complications in people at risk (elderly people or subjects weakened by an underlying chronic pathology). Seasonal flu is responsible for around 10 deaths per year in France.

Viral infections are very contagious. Viruses are transmitted by: 

• Saliva: CMV and Epstein Barr virus (EBV);
• Respiratory secretions during coughing or sneezing: respiratory viruses (rhinovirus, influenza virus, RSV), measles, VZV;
• The skin by the transcutaneous route, by a bite, a bite or via a wound: the rabies virus, HSV, VZV;
• Stools: via food or hands soiled by stool (faecal-oral transmission). Many digestive viruses are present in the stool (adenovirus, rotavirus, coxsackievirus, poliovirus, coronavirus, enterovirus);
• Contaminated objects (manual transmission): Influenzae virus, coronavirus;
• Urine: mumps, CMV, measles;
• Breast milk: HIV, HTLV, CMV;
• Blood and organ donations: HIV, hepatitis B virus (HBV), hepatitis C virus (HCV), CMV…;
• Genital secretions: HSV 1 and HSV 2, CMV, HBV, HIV;

A vector: the virus is transmitted by a bite from an infected animal (yellow fever, dengue fever, Japanese encephalitis, West Nile virus encephalitis and other arboviruses).

Many acute viral infections are asymptomatic (no symptoms) or with general symptoms such as fever, fatigue, and the presence of lymph nodes. This is the case, for example, with rubella, CMV or EBV.

Symptoms of viral infections depend on the infected organ. Many viral infections also give rise to skin symptoms (macules, papules, vesicles, skin rash (redness): this is the case for example of HSV, VZV, rubella for example. Diarrhea, nausea and vomiting are observed during infections with gastroenteritis viruses.

The flu, for example, is manifested by high fever, chills, sneezing, cough, runny nose, intense fatigue, body aches, headaches. Nasopharyngitis (cold) is signaled by fever, stuffy nose, nasal secretions, cough.

Viral infections cannot be treated with antibiotics. Only bacterial complications of viral infections require antibiotics. Viroses are treated for symptoms (fever, pain, cough) with antipyretics or painkillers, or treatments for specific symptoms: anti-emetics in case of vomiting, soothing or moisturizing creams and, sometimes, oral antihistamine for itching caused by certain skin rashes.

Antiviral drugs may be given in severe cases of influenza, to treat HIV, chronic hepatitis B or C, or certain herpesviruses.