Transmission of Infectious Diseases
Stevie-Jade Lisa Vickers – BTEC Nat Diploma in Applied Science Level 3 Medical
What is an infectious disease?
By definition, a disease is a disorder of structure or function in a human, animal or plant, especially one that
produces specific symptoms or that affects a specific location and is not a direct result of physical injury. Diseases
can be communicable or non-communicable. A communicable disease is a disease which can be spread from one
person, animal or plant to another (also known as a contagious disease or infectious disease). Examples of
communicable diseases are HIV or measles.
What causes infectious diseases?
Infectious diseases are caused by organisms including bacteria, viruses, fungi, protozoa and parasites.
How are infectious diseases transmitted?
Air Droplets
When people talk, sneeze or cough, small droplets are released into the air. If infected, these droplets containing
infectious organisms, typically bacteria or viruses, are released into the air. Though these droplets are tiny and are
only able to travel a short distance of around a metre, they can be breathed in by those nearby or land on objects
that others may come into contact with. If someone who has been contaminated by a droplet touches their nose or
mouth, they too can become infected and fall ill. Examples of infectious diseases which can be transmitted via air
droplets are a common cold, the flu (influenza), meningococcal disease and rubella.
The flu is a highly infectious illness caused by a virus called influenza A and influenza B. These viruses cause seasonal
epidemics of illness. An epidemic is the spread of disease in a particular community at a particular time. The flu is a
respiratory illness that infects the throat, nose and sometimes the lungs. The flu, when caught, is often a mild illness,
however, it can become severe and lead to death in some cases. A common cold, also transmitted by air droplets,
has symptoms which develop slowly, whereas the flu comes on suddenly. Common symptoms of the flu include a
fever, a sore throat, a cough, muscle and/or body aches, headaches, a runny nose, tiredness and occasionally nausea
and vomiting. People who are infected with the flu virus are most contagious for the
first 3-4 days after becoming infected. In some cases, adults can infect other people
days before symptoms develop or up to 7 days after becoming ill. Younger children
and more vulnerable people (such as those with weakened immune systems) are
able to infect others for a longer period of time.
When a person infected with influenza virus coughs, sneezes or talks, small droplets
are released into the air. The virus may be present in these droplets and will be
Influenza virus
released into the air too. These droplets, containing influenza virus, can only travel a
under the
short distance, however, they go into another person’s nose. The virus, after being
microscope
inhaled, lands on the cells of the throat. The virus attaches to a receptor site on the
cell membrane of the host cell either through the attachment of viral surface
proteins on the capsid of the virus or by glycoproteins which are embedded in the viral envelope of the virus. This
attachment process is highly specific, and this specificity is what determines the host and the cells within the host
that can be infected by a particular virus, almost like a ‘lock and key’ mechanism. In this case, influenza is able to
attach to the epithelial cells in the throat. When this attachment occurs, the host cell allows the virus entry, and
when the virus enters the cell, it bursts. The contents of the virus are dispersed throughout the host cell including
toxins (which are what cause people to become ill from the virus). The RNA from the virus enters the nucleus of the
host cell where it is replicated. Viral mRNA makes viral proteins and new viral particles are released into the
extracellular fluid until they are engulfed by more cells. The original host cell survives and continues the replication
process, however, some cells which are infected by influenza virus are killed.
The most common and effective preventative method for the prevention of contracting the flu is to receive the flu
vaccination annually. The vaccine has proved to reduce illnesses caused by the flu and to reduce the risk of serious
, flu cases which could potentially end in hospitalization, death or both. Other preventative steps to take include
keeping a distance from those who are infected, covering sneezes and coughs and containing the air droplets
released by these actions by using tissue and frequent handwashing.
Aerosol Transmission / Airborne (Indirect)
Aerosol transmission is similar to transmission via air droplets. Small particles called small particle aerosols are
released when a person talks, breathes coughs or sneezes. If the person is infected, these particles can infect others.
Unlike droplet transmission, small particle aerosols can travel long distances in the air and remain suspended in the
air, without falling, for up to a few hours. Some examples of airborne spread diseases include chickenpox,
tuberculosis and measles.
Chicken pox is a viral infection caused by the virus varicella-zoster. It is
a highly contagious disease, especially to those who have not had the
disease previously or who have not been vaccinated against it. The
varicella-zoster virus causes an itchy rash on the skin which appears
typically between 10-21 days after exposure to the virus. The rash can
last between 5 to 10 days and may be accompanied by symptoms such
as fever, headache, loss of appetite and tiredness. The rash appears as
Varicella Zoster Virus raised bumps (usually red or pink) all over the skin. They then develop
into small blisters filled with fluid which break and leak quite quickly.
The last stage of the rash is when the broken blisters are covered by scabs, which can take days to months to heal.
Though the illness is typically a mild one, the rash can cause lesions to form in the throat, eyes and mucous
membranes of the anus, vagina and urethra.
Chicken pox is airborne, meaning that the virus contained in droplets released when people sneeze and/or cough
remain in the air for a prolonged period of time. The illness can also be transmitted via direct contact with the
infectious rash.
Varicella-zoster is spread by the respiratory route and once an infection is set up in the respiratory tract, the
varicella-zoster virus spreads via the blood, which is why lesions form all over the body. When the virus is inhaled, it
sets down in the lungs and a primary infection begins in the mucosa of the respiratory tract. Once the virus infects
the lungs, it begins to replicate. The replicated cells spread via the bloodstream and lymphatic system and spreads
throughout the entire body and eventually to the skin. Once the body’s defence system fights off the infection, the
virus remains in the nerve cells dormant. If this virus is reactivated, the virus replicates and releases copies of itself
along the neural pathway which infects the skin and once again, causes a rash. The disease then becomes known as
shingles, which is more common in adults than children.
The most common and preventative method for transmitting chicken pox (varicella-zoster virus) is by getting the
chicken pox vaccine which is available. Both adults and children who have either never had chicken pox or who were
never vaccinated should have two doses of the vaccine administered to them. This vaccine has proved to be
extremely effective at preventing the disease and the majority of those who get the vaccine will not go on to
contract the infection. The minority of those who have been vaccinated but still get chickenpox will only develop
mild symptoms. Since the vaccination was developed, the United States has seen a 90% decrease in chickenpox
cases, hospitalizations and deaths, which is evidence of the vaccine’s effectiveness.
