When are we most likely to catch viral diseases?
Viruses have been described as “organisms at the edge of life”,
unable to reproduce outside the cells of those they infect. But this
status has not impeded their evolutionary success. Children, in
particular, experience a multitude of viral illnesses during their early
years, which gradually reduce over time as their natural immunity
Pic - komonews.com
Viral infections may be fleeting (think influenza) or chronic (HIV,
for instance), affecting various parts of the body to cause a diverse
array of symptoms. These differences have important implications for the
spread of that particular viral disease.
Reducing reproduction numbers
The most relevant factor is the infectiousness of a virus, often
summarised in a measure known among epidemiologists as its “reproduction
number”. This describes the average number of secondary infections
produced by one ill individual.
Consider the most commonly transmitted viral illness, the flu: about
10% to 20% of the population is infected by one of the circulating
influenza viruses each year. Symptoms vary from a mild ‘cold’ through to
severe respiratory infection requiring hospitalisation.
So it’s perhaps surprising that the reproduction number of influenza
is relatively low; each infectious person infects only one and a half
other people - or put another way, two infectious people produce, on
average, three new cases of influenza. If we could get that reproduction
number below one, incidence numbers would decline as, on average, each
infected person would not necessarily produce a successor.
What makes the flu difficult to control despite its relatively low
reproduction number is the fact that people are contagious for a day or
two before they show any symptoms. Once they become unwell, they’re
contagious for a couple more days. But by this stage they’re more likely
to stay home and avoid contact with others.
As half or more of their secondary infections will have been produced
before the ill people showed any symptoms, influenza control strategies
that rely on identifying and isolating them aren’t necessarily going to
effectively reduce the incidence of the illness in the community.
This incubation period – the time when a person is infected but not
showing symptoms – is a vital consideration for controlling viral
Consider the outbreak of a deadly strain of the coronavirus that
caused Severe Acute Respiratory Syndrome (SARS) in 2003. Like influenza,
SARS leads to fever and cough, and is mostly spread by close contact
between people. Even though its reproduction number has been estimated
to be between three and four, people carrying the virus are most
contagious in the second week of their infection, after they have
started to show symptoms of the disease.
It was this delay until contagion that made it possible to contain
SARS: health-care workers were able to find and quarantine people who
had contact with the disease before they infected others. Nonetheless,
controlling its outbreak was not an easy task – it spread to around 30
countries and killed about 10% of those infected.
SARS appears to have been eradicated but the recently emerged Middle
East Respiratory Syndrome (MERS), which also belongs to the coronavirus
family, has many similarities. The majority of cases, including the
recent outbreak in the Republic of Korea, have arisen from infection
spread in health-care settings, where early infections are
indistinguishable from other respiratory viruses.
Another virus that’s been grabbing news headlines of late is Ebola.
While flu and SARS spread through coughing and sneezing, contracting
Ebola requires direct contact with the blood or other bodily fluids of
an infected person, or with items such as bedding or clothing
contaminated with these fluids.
As with SARS, people infected with Ebola are not contagious until
they begin to show symptoms, which include diarrhoea, vomiting and
bleeding. Contagiousness increases as these symptoms worsen and peaks
around the time of death. The body of a deceased person remains
contagious after death. Safe burial practices were therefore an
essential component in controlling the recent Ebola outbreak.
The reproduction number of Ebola in the latest outbreak was as high
as four. But, as with SARS, the delay until contagion made it possible
to trace and quarantine people who had been in contact with Ebola before
they spread the disease any further.
The importance of vaccines
In stark contrast to these stories of successful disease control, the
World Health Organisation estimates that 35 million people are living
with human immunodeficiency virus (HIV) infection. And more than 39
million have died from the infection to date.
This toll is largely due to the fact that people with HIV infection
may take anywhere between two and 15 years to develop symptoms of
acquired immune deficiency syndrome (AIDS). Meanwhile, they unknowingly
infect others over a period of many years, rather than days or weeks.
HIV spread can be effectively reduced by antiretroviral drugs, which
lower virus levels in the blood. Unfortunately, WHO estimates that only
a third of the people who need these drugs currently have access to
them. Meanwhile, the quest for an HIV vaccine continues. Vaccines, in
fact, are one of the best ways to control the spread of viral illnesses.
Consider the numerous childhood illnesses that are now contained thanks
Measles and chicken pox, for instance, are classically identifiable
by their distinctive rashes. But they’re most infectious in the time
between the appearance of non-specific symptoms, such as fever, runny
nose and cough, and the development of this rash. This made it very
difficult to control their spread until vaccines were developed.
- The Conversation