Viruses are for sure simple microorganisms in regards of cellular complexity , yet , when it infects a complex organism such as human beings , it can cause severe complications and in some cases death. Hence, it is fairly reasonable to ask yourself ,how viruses cause disease ,also known as the “viral pathogenesis“ which we will be discussing in this post. Famous viral infections include Influenza , Measles, Herpes Zoster, Hepatitis B, AIDS, Yellow Fever and the list goes on and on.

HOW VIRUSES CAUSE DISEASE

To understand how viruses cause disease we must first set some ground rules about viruses. A virus is a pathogenic organism for a particular host . It depends on the virulence of the virus to actually cause a disease to a particular organism. However, for a virus to cause disease for a specific host, it first has to have a special criteria to infect this specific host. This is called Virus Tissue Tropism. An example for this is the HIV which has specific viral attachment proteins on its surface ( gp 120) in order to only bind to T Lymphocytes of the immune cell which has CD4 receptors (in figure below). Furthermore, viruses cause disease depending on the host status, for example, Herpes Simplex virus causes different complications in different hosts ; in immunocompetent individuals it causes cold sores while in immunocompromised individuals it causes Pneumonia .

Now that we have established some ground rules about viruses, let’s discuss thoroughly how viruses cause disease.

In order for a virus to cause a disease or an infection for multi-cellular organisms , it has to fulfill 4 requirements.

Check this quick Infographic about the 4 steps of viral pathogenesis

The 4 steps are :

PORTAL OF ENTRY

TARGET TISSUE IN WHICH TO REPLICATE

PORTAL OF EXIT

MEANS OF TRANSMISSION

So to explain what causes a viral infection, we should first explain each step of the viral infection requirements listed above and its significance.

A) PORTAL OF ENTRY

This step is the first step of the viral infection journey ( Virus Pathogenesis) .This is concerned with how the virus enters the host’s body. The portal of entry can also be the place where the virus causes the disease which is the target tissue which we will discuss in the second bullet point.

Viruses can enter the host’s body through several portals of entry.

Mucosal – such as through Conjunctiva (Adenovirus), Oropharynx (Herpes Simplex virus), Respiratory tract (Influenza & Measles), Alimentary tract (Poliovirus), Genital tract (HIV, Hepatitis B virus, paillomavirus)

– such as through Conjunctiva (Adenovirus), Oropharynx (Herpes Simplex virus), Respiratory tract (Influenza & Measles), Alimentary tract (Poliovirus), Genital tract (HIV, Hepatitis B virus, paillomavirus) Skin – Papillomavirus (Common Warts)

– Papillomavirus (Common Warts) Blood – HIV, Hepatitis B virus, Yellow fever virus (Biting Arthropods)

– HIV, Hepatitis B virus, Yellow fever virus (Biting Arthropods) Placenta – Congenital Rubella

B) TARGET TISSUE FOR REPLICATION

After the virus ‘decides’ its portal of entry it then either infects the portal of entry (Localized Infection) or travels and infects a site distant from the portal of entry (Systemic Infection)

Examples for Localized Infection : Influenza Virus, Rhinovirus, Rota virus, Papilloma virus

Examples for Systemic Infection : Polio virus, Measles virus

The virus then binds with the target cell surface and starts to fuse with the cell membrane pouring its contents (viral genome) into the cell. This process is called Unpacking. The viruses’ genetic material first replicates into many copies. Viral genome – either DNA or RNA or both – then uses the cell’s components (ribosomes) to transcribe it’s genetic material into proteins. Viruses’ genetic material copies along with the viral proteins synthesized with host cell components assemble and get ready to leave the cell to infect other host cells with the same mechanism. This is how viruses cause disease.

C) PORTAL OF EXIT

After millions and millions of viruses have assembled it then exits the cell or even bursts the host cell open and infect neighboring cells. Viruses can also reach outside the whole body and go to infect other individuals. Viruses can leave the body through one site or multiple sites.

Examples of virus exit portals:

Respiratory Tract : Influenza, Measles

Oropharynx : Herpes Simplex Virus

Alimentary Canal : Polio Virus

Genital Tract : HIV, Hepatitis B

Blood : Yellow Fever Virus, HIV, Hepatitis B

D) MEANS OF TRANSMISSION

This topic is concerned with how the virus transmits from one individual to another. Virus transmission depends on the amount of virus shed, duration of shedding, and the ability of the virus to survive in the environment until it reaches the host body and infects it.

Types of transmission

Vertical Transmission

Parent to embryo/fetus/newborn

Horizontal Transmission

Between individuals

Latrogenic Infection

Acquired by patient during treatment

Nosocomial Infection

Acquired by patient in hospital

Virus transmission can be Direct or Indirect.

Direct: Virus transmission from infected person/animal to susceptible host. Direct transmission by skin, tissue, blood, sexual contact, congenital/trans-placental, perinatal, respiratory secretions.

Indirect: Virus transmission from infected person/animal to susceptible host. Indirect transmission by airborne-inhalation, vehicle-borne (fomite), vehicle-borne (faecal/oral route), vehicle-borne (blood), vector-borne.

PREVENTION OF VIRUS TRANSMISSION

GENERAL HYGIENE, HAND WASHING, BARRIER NURSING/DISINFECTION,AUTOCLAVES PROTECTED SEX FOOD & WATER HYGIENE ARTHROPOD VECTOR CONTROL VACCINES PROTECTION OF SUSCEPTIBLE HOSTS BY POST EXPOSURE PROPHYLAXIS (PEP) LONG TERM VACCINATION

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