1. VIRAL DISEASES AND EPIDEMIOLOGY
Claude MUVUNYI M.D., Ph.D.

2. Viral diseases
The relative susceptibility of a person and the severity of the disease depend on factors such as:
the nature of exposure;
the immune status,
age, and general health of the person;
the viral dose and
the genetics of the virus and the host

3. Therefore:
new variant or viral strain would cause a viral outbreak (new influenza virus strain).
acute A hepatitis is more symptomatic in adulthood than in childhood.
Infants are especially prone to more serious presentation of Paramyxovirus respiratory infections and gastroenteritis.
However, children generally do not mount as severe an immunopathological response as adults, and some infections (Varicella caused by VZV a Herpesvirus) are more benign in children.
The elderly are especially susceptible to new viral infections and reactivation of latent viruses.

4. Stages of the viral disease
The initial period before the characteristic symptoms of a disease is termed the incubation period.
During that period, the virus is replicating but has not reached the target tissue or induced sufficient damage to cause disease.
The incubation period is relatively short if the infection of primary site produces the characteristic symptoms of the disease; e.g., 1-2 days for influenza.
This incubation period is longer for viral systematic infections such as
poliomyelitis ( 5-20 days),
measles (9-12 days),
rubella (17-20 days),
hepatitis (15-40 days for hepatitis A and days for hepatitis B),
rabies ( days),
AIDS for Acquired Immunodeficience Syndrome cause by HIVs (1-13 years).

5. Types of viral infections
Viral infections are usually self-limiting. Sometimes, however, the virus persists for long periods of time in the host.
Long-term virus-host interaction may take several forms:
1- Chronic infections.
2- Latent infections.
3- Inapparent or subclinical infections.

6. Transmission ways of viruses
Viruses are transmitted
by direct contact,
sexual intercourses,
injection with contamined fluids or blood,
use of contamined injection materials (syringues, catheters), or prosthetic devices,
and the transplantation of noncontrolled contamined organ,
more often by respiratory and oral-fecal routes.
They may also be transmitted by a maternal-neonatal transmission during foetal live or the delivery.

7. Transmission ways of viruses
Animals can also act as vectors that spread viral infection to other animals and humans.
They can also be reservoir for the virus that maintain and amplify the virus in environment.
Viral infections that are shared by animals or insects and humans are called zoonosis.
Arthropods include mosquitos, ticks can act as vectors for Togavirus, Flavivirus, Bunyavirus and Reovirus.
These viruses are often referred to as arboviruses because they are arthropod borne viruses, such as Yellow fever virus, Dengue viruses, Rift Valley fever virus.

8. Transmission ways of viruses
MODE
VIRUSES
Respiratory transmission
Paramyxoviruses, influenza viruses, picornaviruses, Varicella-zoster virus.
Fecal oral transmision
Picornaviruses, rotaviruses, Reovirus, calicivirus, Norwak virus,adenovirus
Contact (lesions, saliva, fomites)
Herpes simplex viruses, poxviruses, adenoviruses
Zoonoses ( animals, insects)
Togaviruse, flavivirues, bunyaviruse, arboviruses, arenaviruses, rabies virus
Transmission via non-controlled blood, fluids
HIV, HTLV-1, hepatitis B,C and D virus, cytomegalovirus
Sexual contact
Blood-borne viruses, herpes simples 2, humanpapilloma virus
Maternal-neonatal transmission
Rubella virus, cytomegalovirus, B 19 virus,echovirus, herpes simplex virus 2, Varicella-zoster virus, HIVs

9. how viruses leave the body
Infectious agents leave body by many ways: 
1° with excreted products from epithelial cell mucous; with measles infection, conjunctival excretions; rhinopharyngeal excretions of flu infection; bronchal excretions of viral pneumonia.
2° with glandular excretions for mumps and saliva for rabies
3° with feces containing Enterovirus and other gastrointestinal viral infectious agents such as oral fecal transmitted hepatitis viruses A and E
4° with fuids of cutaneous viral lesions such as vesicles or pustules that contain infectious agents(VZV).
All these excreted products are potential infectious; they are, also mainly used as specimens for biological diagnosis of viral infections and diseases

10. Viral infection epidemiological types
We account three epidemiological types:
sporadic,
epidemic,
pandemic

11. Sporadic Concern one person or another with any relationship.
It cannot spread in all the population.
Those persons with sporadic infection act as reservoir if the infection may be persitent, latent or chronic:
Example is that HBV chronically people infected form a reservoir of the virus and establish the endemicity of that infection.

12. Outbreaks or epidemies
often occurs from the introduction of a virus in a new location.
The outbreak comes from a common source (e.g.,food preparation) or a new strain in a immunological naïve population, (new influenza virus strain).
Epidemy concern a wide population during the same time and in a location (e.g., influenza school outbreak)

13. Pandemic
are world-wide epidemics such as HIV 1 pandemy and Influenza A virus pandemy.
These epidemies spread from one continent to an other and may concern in a short time population of all the world
Pandemies of Influenza virus A have occurred approximately each 10 years as result of the introduction of a new strain .

14. Current viral infections and diseases
Congenital Viral Infections.
Foetal and neonatal viral infections
Childhood viral infections and diseases
Acute Viral Respiratory Infections.
Viral Infections of the Gastrointestinal Tract.
Viral skin Infections
Viral Infections of the CNS

15. Main foetal and neonatal viral infections
Rubella virus causes in utero infections
Cytomégalovirus with in utero and peri natal infections
→Those two foetal infections would be transferred through placenta from infected mothers; they are referred main severe congenital infections
Herpès simples 2, with perinatal infections,
Varicella and zoster virus that cause in utero and perinatal infections
B 19 Parvovirus cause in utero infections.
HIVs can cause in utero and peri-natal infections,
HBV may cause peri-natal infections

16. Main Childhood viral infections diseases
During childhood, viral infections are dominated by exanthemous and other cutaneous manifestations. We can mention:
Measles caused by measles virus, a RNA virus, belonging to genus Morbillivirus of Paramyxoviridae family ,
Rubella caused by a RNA virus, belonging to genus Rubivirus of Togaviridae family,
Varicella which is essential a benign disease with very rare complications in immunodeficienty children. It is caused by Varicella and Zoster virus or VZV ou HHV 3, DNA virus belonging to Varicellovirus genera of Alphaherpesvirinae sub-family from Herpesviridae family. This virus may cause meningitis by immunodeprissive children
Mumps virus is responsible of parotiditis, a childhood benign disease. That’s a RNA virus of Paramyxoviridae family. It can sometimes cause neurologic disorders such as meningitis or meningo- encephalitis.

17. Respiratory tract viral infections
Those called respiratory virus share an exclusive tropism for respiratory epithelial ciliated cells. We account with:
flu Influenza A and B
Para-influenza 1, 2, 3 and 4
Respiratory syncytial virus or R.S.V
respiratory Adenoviruses
Rhinovirus,
Coronavirus.
Human Metapneumovirus
Each group of virus may cause different respiratory syndroms such as cold, laryngitis, tracheitis, bronchitis, bronchiolitis, pneumonia or flu.
Each clinical syndrome may be characteristict of a specific group of viruses. Such as
cold caused by Rhinovirus,
laryngitis by Parainfluenza virus,
bronchiolitis by Syncitial respiratory virus
flu by influenza virus.
But also different viruses can cause a same clinical syndrom

18. Main gastro-intestinal tract viral infections
We deal with hepatitis and gastroenteritis
Hepatitis causative agents may be A, B, C, D and E virus with « new hepatitis virus »GB-A, GB-B, GB-C/VH-G et TTV.
All these viruses belong to different families

19. Hepatitis causative agents
Virus
Family
Genome of
Types of transmission
HVA
Picornaviridae
Heparnavirus
RNA
Enteral ou oro-fecal
HVB
Hepadnavirus
DNA
Parenteral, mother-
neonatal
sexually
HVC
Flaviviridae
Parenteral
HVE
Calciviridae
Enteral or oro-fecal
GB-A
GB-B
GB-C/VHG
TTV
Parvovirus

20. Gastroenteritis causative agents
Those viruses are:
Rotavirus
Astrovirus
enteric Adenovirus serotypes 39, 40
Norwack agent
Coronavirus
All these gastroenteritis associated viruses are not detected at same frequency.
Rotavirus and new enteric Adenovirus are the more frequently detected from newborn gastroenteritis. Others interest children and adults are rarely isolated.
A new vaccine is available against Rotavirus

21. Central nervous system viral infections
These acute clinical syndromes may be:
Aseptic meningitis or AAP for Acute anterior poliomyelitis that is caused by Poliovirus;
human or animal rabies caused by Rabies viruses
Four CNS chronic and degenerative diseases are associated with conventional agents. These diseases include:
Sclerotic sub-acute pan-encephalitis which may be caused by Measles virus
Progressive multiform leucoencephalitis due to J.C. virus a Papovirus
Tropical spastic paralysis (T.S.P) due to human T leukemia virus type 1 (HTLV1) an Oncornavirus belonging to Retroviriday family
AIDS dementia associated to Human Immunodeficiency Virus type 1 or 2, HIV 1 and 2, species of genera Lentivirus and Retroviridae family