As the throat, preventing them from wafting mucous,

As well as this there are many complex factors that can make things
worse. Smoking, for example creates a hot smoke that paralyses the cilia in the
throat, preventing them from wafting mucous, a mechanism, which usually acts as
a barrier against microbes1. Numbers of smokers have admittedly been
declining in the past century now that the public has been made aware of its
consequences, however,
__% of the population still smokes. Another factor is immunosuppression,
which a number of diseases like HIV can cause. A weakened immune system of a significant amount of a
population could potentially cause a disease to spread rapidly through a
population1. An example of the importance of the immune system can
be seen with David Vetter, a boy born with severe combined immunodeficiency
(SCID), which forced him to be kept in a protective bubble from his birth in
1971 to his death in 1984, any exposure to non sterile conditions would have
proven fatal.

 

Aside from the
body’s defense mechanisms, a major worry nowadays, in terms of spread of
pandemics, is definitely the current interconnectivity in our modern society. It
now only takes hours to travel the world compared to the many weeks in the past
century (see Fig. 1). Naturally this
puts all of humanity at risk of deadly disease- we are all in a ‘Hot Zone’1.

However, short flight times and air filtration systems on jetliners protect
most from infections such as TB1. Possibly more concerning is the
fact that even if we stopped all flights to the UK we would only stop the
arrival of influenza by about 4 weeks3, as calculated by our best mathematical model.

This factor of immigration and people crossing boarders without the
country’s permission/knowledge is an interesting one.  Immigrants, who have exposed themselves to possibly
rare tropical diseases, are unlikely to seek medical care on account of legal
or financial consequence (and sometimes the government refuses to treat them,
like in America in the last century)1. This makes them very
difficult to target as a population especially in combination with ease of
travel, upon entry, and the fact that they are more likely to travel and thus
more likely to spread disease. Furthermore, a lot of illegal immigrants also
work in the food industry1- a utility that many people utilise. This
ties in with people’s psychology and how this affects the spread of disease. In
most cases when people become aware of their infection they change their
behavior and avoid contact and exposure with and to others4. This
can be seen as a positive factor that reduces a pandemics spread. However,
there are also negative implications of behaviors, for example some cultures
mean that people are less likely to complain about a disease and severe
symptoms such as hemoptysis (coughing up blood)1.

 

Another major
factor of the spread of a disease is how close people are to each other on
average, in other words population density. As shown in Fig. 2 there is a trend of urbanisation: we are getting more
densely populated cities as population increases. Population density is in
combination with poor sanitation in many of the world’s slums, while the
percentage of people living in slums has gone down it is still high in some
parts of the world.  In fact, about 1
billion people in developing countries live in slums. Furthermore, there is roughly
one toilet for every 500 people in the slums in Nairobi, Kenya5.

This means that for a disease that is transmitted for example via the
faecal-oral route, there are a potential of 500 people that could be infected
from a single source, which dramatically helps a disease’s initial rate of
infectivity.

 

Another factor that has to be addressed is the latency period and
incubation period relationship. The latency period being the time to
infectiveness and the incubation period being the time until the appearance of
symptoms, both from the point of infection6. Different diseases have
different length for these times. For example the bacterium causing
tuberculosis can take weeks to years to show symptoms1 whereas
Influenza causes disease after 1-3 days6. It is clear
that if a disease could lay hidden for a long period of time while also being
infective then it would be both hard to find infected people in a population
and disease carriers wouldn’t take actions to isolate themselves from others.

Diseases such as measles or chickenpox are examples in which, in the last two
or three days of incubation they are communicable7. Generally, a
shorter incubation period means a more acute and severe illness (longer being
less severe), so it is likely that our ‘world ending pathogen’ would have a
short incubation period – to kill more people7.

 

Finally, there is a significant risk that an animal strain of flu could
mutate and become transmissible between humans potentially killing millions3.

A mutated strain would potentially be more deadly, something that will be
looked at later on in this essay. This conclusion can be made from the
‘Trade-off’ hypothesis’ that states, from an evolutionary aspect, pathogens are
forced to make certain compromises8. The most notable of these is
between transmission and virulence (how deadly the disease is), that in order
to spread more efficiently a disease must evolve a lesser virulence to kill
fewer hosts8. Thus one can see that it is less likely that an
endemic disease would be responsible for an extinction level pandemic. However,
this theory has its flaws and is yet to be fully justified. For example in 1950
a biological control agent, the myxoma virus, was released into Australia to
kill rabbits- this disease attenuated (lost virulence) thus reinforcing the
hypothesis. However, in 1995 the Rabbit Haemorrhagic
Disease Virus, released for the same purpose, actually increased in virulence
through time.