The diseases which are caused by
microorganisms such as viruses, bacteria, fungi or parasites are called
Infectious diseases and can spread between individuals.
Microorganisms that cause disease are
collectively called pathogens. Pathogens cause disease either by disrupting the
bodies normal processes and/or stimulating the immune system to produce a
defensive response, resulting in high fever, inflammation and other symptoms.
Infectious diseases can be spread
from one person to another, for example through contact with bodily
fluids, by aerosols (through coughing and sneezing), or via a vector, for
example a mosquito.
Infectious diseases are one of the
leading causes of death worldwide. Many diseases become difficult to control if
the infectious agents evolve resistance to commonly used drugs:
For example, bacteria can accumulate mutations in their DNA
or acquire new genes that allow
them to survive contact with antibiotic drugs
that would normally kill them.
Scientists are currently searching
for new approaches to treat infectious diseases, focusing on exactly how pathogens change and drug resistance evolves.
What causes an infectious disease?
Viruses
Viruses are tiny infectious agents
that replicate only in the living cells of other organisms. They have a very
simple structure consisting of genetic material in the form of DNA or RNA within a protein capsule. They can infect all types of life
forms, from animals to plants and bacteria to amoebae.
Viruses can be spread in many ways
including:
·
From plant to plant by insects that feed
on plant sap. For example, Potato virus Y which is spread by aphids.
·
From animal to animal by blood-sucking
insects. For example, Dengue virus which is spread by mosquitos.
·
Spread by aerosols (through coughing and
sneezing). For example, influenza virus, COVID-19 (Corona Virus Disease 2019).
·
Spread by not washing hands after going to
the toilet. For example, norovirus or rotavirus.
·
Spread by sexual contact. For example, HIV and Human
Papillomavirus (HPV).
·
Spread by exposure to infected blood. For
example, Hepatitis B, HIV.
·
Spread by exposure to fluid, such as seat,
saliva, etc. from infected body (Human or other animals). For example, Ebola
Virus, Corona Virus.
·
Viruses can often be prevented
through vaccines to boost the body’s immune
system.
Bacteria
Bacteria are single-celled
microorganisms. They come in many shapes including ball-, rod- and
spiral-shaped. Most bacteria are not harmful and some are actually beneficial.
Less than one per cent of bacteria will actually make you ill.
Infectious bacteria can grow, divide
and spread in the body, leading to infectious disease. Some infectious bacteria
give off toxins which can make some diseases more severe.
Bacteria are spread in many ways
including:
·
Spread by aerosols (through coughing and
sneezing). For example, Streptococcus.
·
Spread by surface and skin contact. For
example, Staphylococcus aureus, including MRSA.
·
Spread through body fluids, such as blood
and saliva. For example, meningococcal disease (meningitis).
·
Spread by aerosols (through coughing and
sneezing). For example, cough, Tuberculosis (TB).
·
Spread by not washing hands after going to
the toilet. For example, cholera, dysentery.
Antibiotics are usually given to treat severe
bacterial infections.
Antibiotic resistance in bacteria is
a significant problem.
Streptococcus bacteria. Image credit: Shutterstock |
Fungi
Fungi are microorganisms
characterised by cell walls made from a substance called chitin. Most fungi are
harmless to humans and some are edible. Other fungi can be infectious and may
lead to life-threatening diseases.
Fungi reproduce by releasing spores
that can be picked up by direct contact or even inhaled. Fungal infections
often affect the lungs, skin or nails. Some infections may also penetrate the
body to affect organs and cause whole-body infections.
Examples of fungal infections
include:
·
Athlete’s foot: itching,
scaling or cracking of the skin
·
Ringworm:
reddish, itchy, scaly rash usually on the skin and scalp
·
Thrush:
caused by the fungus Candida albicans which can infect the mouth,
vagina, stomach and urinary tract.
Parasites
Parasites are organisms that live in
or on another organism and benefit by getting nutrients at the expense of their
host.
Parasites can be found in many
different body sites, for example in the blood, liver, digestive system,
brain and even the eyes. There are several groups of parasites that infect
humans:
·
Elephantiasis lymphatic filariasis:
This is transmitted through mosquito bites.
The adult worms live in the lymph system. Infection can lead to lymphoedema and
elephantiasis, in which swelling can cause disfigurement and disability. In the
Americas, it is passed on by the Culex
quinquefasciatus mosquito.
·
Protozoa:
these are single-celled parasites. Many
are delivered by biting insects or are ingested with contaminated water or
food. They invade or eat host cells, or swim around the bloodstream and
quickly reproduce by copying themselves. Examples: Giardia (ingested), Plasmodium
falciparum (causes malaria and is transmitted by mosquitos), Trichomonas (sexually
transmitted)
Worms
Worms, or helminth organisms, can
affect humans and animals.
·
Tapeworms:
these are flatworms whose heads hook on to the intestinal wall of their host.
Their larvae migrate to other parts of the
body. They can live in the host for decades. Example: Pork tapeworm.
·
Screwworm: This is transmitted by a fly, and it
affects skin and wounds. It is found in Central America and North Africa.
·
Beef and pork tapeworms:
Taeniasis is caused by tapeworms of the taenia family. They
affect the intestines. They are passed on by eating undercooked beef or pork.
Different types of tapeworms can affect the intestines, the liver, or the lungs.
·
Toxocariasis:
A roundworm transmits this infection from
animals to humans. It affects the eyes, brain, and liver. It is caused by
accidentally swallowing the eggs of the parasite, for example, when young
children play with soil. Nearly 14 percent of people in the U.S. have
antibodies, suggesting that millions have been exposed. Most never have
symptoms.
·
Trichinosis:
This is caused by the roundworm of
the Trichinella family. Infection can lead to intestinal symptoms, fever, and
muscle aches. It is passed on by eating the undercooked meat.
·
Whipworm:
Also known as trichuriasis, whipworms
live in the large intestine. Eggs are passed in faeces. It is common all over
the world. Humans can become infected when ingesting eggs, for example on
unwashed fruit or vegetables.
·
Anisakiasis:
This is caused by worms that can invade the intestines or the stomach wall.
The worms are passed on through
contaminated fresh or undercooked fish and squid.
·
Roundworms:
Ascariasis, or a roundworm infection, these are nematode worms that can get
into humans by ingestion, directly penetrating the skin or through insect
bites. Examples: whipworm (ingested), hookworms (directly
penetrate the skin), and Wuchereria bancrofti (transmitted by
mosquitos). a roundworm infection does not usually cause symptoms, but the worm
may be visible in faeces. It enters the body
through consuming contaminated food or drink.
Roundworms can be passed on by raccoons. |
· Hookworm: These can cause intestinal disease. They lay their eggs in the soil and the larvae can penetrate the skin of humans. Early symptoms include itching and a rash. They are most common in damp places with poor sanitation.
Hookworms can cause intestinal disease |
· Flukes: blood flukes are flatworms that are caught through contact with contaminated water. Blood fluke larvae are released from infected snails into freshwater. Liver flukes can be found in raw or undercooked fish or water plants and can live in the body of the host for decades. Example: Schistosomes (blood flukes).
Male and female schistosomes. The female can be seen lying within a groove on the surface of the male (stained pink). Image credit: Shutterstock |
Prevention
of parasites-related diseases
To increase your chance of avoiding
parasites:
- find out
which kind are prevalent in your area or in locations you may travel
- take
precautions, for example, using insect repellent in places where
mosquitoes are common
- be careful to
eat only well-cooked fish and meat
- when
traveling, drink only water from bottles with a sealed top
- take care
when bathing in fresh-water lakes or rivers
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