There are several factors that may contribute (individually or in combination) to causing an epidemic. There may be changes in a
pathogen, in the population that it can infect, in the environment, or in the interaction between all three. Factors include the following:
SARS-CoV2 has demonstrated antigenic drift and possibly shift as well.
Drug resistance Antibiotic resistance applies specifically to
bacteria that become resistant to
antibiotics. Resistance in bacteria can arise naturally by
genetic mutation, or by one species acquiring resistance from another through
horizontal gene transfer. Extended use of antibiotics appears to encourage selection for mutations which can render antibiotics ineffective. This is especially true of
tuberculosis, with increasing occurrence of
multiple drug-resistant tuberculosis (MDR-TB) worldwide.
Changes in transmission Pathogen transmission is a term used to describe the mechanisms by which a disease-causing agent (virus, bacterium, or parasite) spreads from one host to another. Common modes of transmission include: - •
airborne (as with influenza and COVID-19), •
fecal-oral (as with cholera and typhoid), •
vector-borne (malaria, Zika) and •
sexual (syphilis, HIV) The first three of these require that pathogen must survive away from its host for a period of time; an evolutionary change which increases survival time will result in increased virulence. Another possibility, although rare, is that a pathogen may adapt to take advantage of a new mode of transmission
Seasonality Seasonal diseases arise due to the change in the environmental conditions, especially such as humidity and temperature, during different seasons. Many diseases display
seasonality, This may be due to one or more of the following underlying factors: - • The ability of the pathogen to survive outside the host - e.g.
water-borne cholera which becomes prevalent in
tropical wet seasons, or influenza which peaks in temperate regions during winter. • The behaviour of people susceptible to the disease - such as spending more time in close contact indoors. • Changes in immune function during winter - one possibility is a reduction in vitamin D, and another is the effect of cold on mucous membranes in the nose. • Abundance of vectors such as mosquitoes.
Human behaviour '' (1852). Changes in behaviour can affect the likelihood or severity of epidemics. The classic example is the
1854 Broad Street cholera outbreak, in which a cholera outbreak was mitigated by removing a supply of contaminated water - an event now regarded as the foundation of the science of
epidemiology. Urbanisation and overcrowding (e.g. in
refugee camps) increase the likelihood of disease outbreaks. A factor which contributed to the initial rapid increase in the
2014 Ebola virus epidemic was
ritual bathing of (infective) corpses; one of the control measures was an education campaign to change behaviour around funeral rites.
Changes in the host population The level of immunity to a disease in a population -
herd immunity - is at its peak after a disease outbreak or a vaccination campaign. In the following years, immunity will decline, both within individuals and in the population as a whole as older individuals die and new individuals are born. Eventually, unless there is another vaccination campaign, an outbreak or epidemic will recur. It's also possible for disease which is endemic in one population to become epidemic if it is introduced into a novel setting where the host population is not immune. An example of this was the introduction European diseases such as smallpox into indigenous populations during the 16th century.
Zoonosis A
zoonosis is an
infectious disease of humans caused by a pathogen that can
jump from a non-human host to a human. Major diseases such as
Ebola virus disease and
salmonellosis are zoonoses.
HIV was a zoonotic disease transmitted to humans in the early part of the 20th century, though it has now evolved into a separate human-only disease. Some strains of
bird flu and
swine flu are zoonoses; these viruses occasionally recombine with human strains of the flu and can cause
pandemics such as the
1918 Spanish flu or the
2009 swine flu. ==Types==