Infectious Diseases

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Pathogens and disease

Infectious disease is a major cause of death worldwide. It is estimated that 14.7 million people died in 2002 due to infectious diseases.

An infectious disease is a disease resulting from infection of a host organism by a pathogen, a disease-causing organism.

There are several different types of pathogen including bacteria, fungi, viruses and parasites.


Impact of infectious diseases


Different types of pathogens


Mechanisms of disease

damaging cells – for example, malarial parasites cause disease symptoms by bursting red blood cells.

For example, the symptoms of cholera are caused by an exotoxin secreted by the bacteria Vibrio cholerae. It increases the secretion of chloride ions into the lumen of the intestines, which can lead to severe diarrhoea.

producing toxins – exotoxins are secreted by a living, normal pathogen, whereas endotoxins are released when the pathogen is damaged.

Pathogens can cause disease in two main ways:


Disease transmission


Factors affecting the spread of disease

Contributing factor(s) Example infectious diseases

Human demographics and behaviour

Economic development and land useMicrobial adaptation and change

Breakdown of public health measures

Climate change

International travel

dengue fever, sexually-transmitted diseases Lyme disease, malaria, plague, rabies, yellow fever

influenza, HIV/AIDS, malaria,

S. aureus infectionsrabies, tuberculosis, cholera, diphtheriamalaria, dengue fever, cholera, yellow fever

malaria, cholera


What is epidemiology?

Epidemiology is the study of disease origin and spread. It covers various aspects, including:

understanding the causation of disease

explaining patterns of disease

investigating the natural history of disease

public health planning.

Epidemiologists collect and analyse statistical data to understand more about existing and emerging diseases.


Incidence, prevalence and mortality


Scientist case study



What are HIV and AIDS?

HIV is a retrovirus that causes acquired immunodeficiency syndrome (AIDS) – a deterioration of the immune system.

In 2007, 33.2 million people were estimated to be living with the human immunodeficiency virus (HIV).

There is currently no cure for infection with HIV; however, antiretroviral drugs have been developed to help delay the onset of AIDS.


HIV replication


Effect of HIV on the immune system


Stages of HIV infection

Replace circles with outline of HIV taken from animation


Antiretroviral drugs

Modern antiretroviral drugs are designed to reduce the production of HIV by targeting different stages of its lifecycle.

reverse transcriptase inhibitors prevent viral RNA being copied into DNA for protein synthesis.

HIV can develop resistance to these drugs so they are often taken in combination.

protease inhibitors inhibit proteases used in the synthesis of viral proteins.


HIV diagnosis and transmission


HIV and AIDS



What is malaria?

Malaria is a disease caused by a protozoan parasite.

There are several species of the parasite, including Plasmodium vivax, P. malariae and P. falciparum, the latter being is the most common and the most dangerous type.

Approximately 40% of the world’s population, mostly those living in the world’s poorest countries, are at risk of malaria. Without treatment, malaria can be life-threatening.

Malaria is transmitted bya vector – the Anopheles mosquito.


What is parasitism?

Parasitism is a close and permanent association between two organisms in which one benefits and the other is harmed.

Parasites can be classified into:

endoparasites – these live within the body of a host, e.g. tapeworm.

ectoparasites – these live on the surface of a host, e.g. ticks.

Some parasites are pathogens that cause diseases in humans, such as malaria, African sleeping sickness,toxoplasmosis and giardiasis.


Symptoms of malaria

They typically occur cyclically every two to three days, depending on which species of Plasmodium is involved.

Young children are especially vulnerable due to their immature immune systems.

The symptoms of malaria include shivering, fever, sweating, joint pain, vomiting and anaemia.

If untreated, severe malaria can lead to spleen and liver damage, renal failure, coma and death.


The malarial parasite life cycle


Control of malaria

In the 1960s the incidence of malaria was greatly reduced by the use of insecticides and drugs. However, the evolution of resistance in mosquitoes and plasmodium species has caused a resurgence of the disease.

A genetically-modified, malaria-resistant strain of mosquito has also been developed. How might this help control malaria?

Currently, different methods of control and prevention are used. The Anopheles mosquito can be controlled using insecticides, environmental management and biological control.


malarial parasites in the mosquito develop quicker at higher temperatures

the life span of a mosquito is linked to temperature and humidity

mosquitoes need water to breed.

The distribution of malaria

Malaria is therefore most common in warm, wet regions. How might climate change affect the number of people at risk from malaria?

Malaria is most common in the world’s equatorial regions. The prevalence of malaria is linked to climate, because:


Climate change and malaria


Life cycle of malaria



What is tuberculosis?

Tuberculosis (TB) is caused by the airborne bacteria Mycobacterium tuberculosis. It is a widespread disease that affects the respiratory system.

One third of the world’s population is currently infected with the bacterium.

TB can be treated using a course of antibiotics; however, antibiotic resistance strains of TB have emerged.


Tuberculosis infection

When a person becomes infected with TB, an immune response is produced.

However, the bacteria can survive inside macrophages for years until the immune system becomes weak.

In healthy people, white blood cells called macrophages engulf the bacteria by phagocytosis, controlling the infection.

A tissue mass called a tubercle forms around the infected site, and after 3–8 weeks the infected region heals.


Symptoms of tuberculosis

A patient with active tuberculosis may experience symptoms such as coughing, shortness of breath, loss of appetite, weight loss, fever, night sweats and extreme fatigue.

Why might an increase in temperature be harmful?

Fever and night sweats occur due to neutrophils and macrophages releasing fever-causing substances, as part of the inflammatory response. These chemicals affect the hypothalamus and increase core body temperature.

If the patient’s immune system is unable to contain the infection, active tuberculosis may occur. The bacteria will multiply rapidly, destroying the lung tissue, which can be fatal.


‘Cause’ of tuberculosis?


Tuberculosis: true or false?



What are antibiotics?

Antibiotics are a group of drugs used to treat bacterial infections.

bactericidal – kill bacterial cells

bacteriostatic – slow the growth or reproduction of bacteria.

They are effective against prokaryotic cells but they generally leave eukaryotic cells unharmed.

Some antibiotics prevent the formation of bacterial cell walls, resulting in osmotic lysis.

There are two different types of antibiotics:


Testing antibiotics


Antibiotic resistance

In the presence of an antibiotic, bacteria with resistance to that antibiotic (due to genetic mutation) have an advantage.

M. tuberculosis has evolved resistance to streptomysin and other antibiotics. Multi-drug resistant TB is still relatively rare, but there is growing concern about its spread.

They are able to survive and reproduce, giving rise to a population of antibiotic-resistant bacteria. In some cases this resistance is evolving faster than the development of new antibiotics.


Antibiotic resistance: MRSA

The bacteria Staphylococcus aureus has become resistant to most antibiotics, including methicillin. This methicillin-resistant S. aureus (MRSA), which is becoming common in hospitals, can cause a life-threatening infection.

Due to the emergence of antibiotic resistance, several methods of prevention and control have been adopted: antibiotics should only be prescribed when needed

patients should finish the complete course of antibiotics

introduction of infection control in hospitals.


Antibiotic resistance worldwide

pathogen drug country/region% of drug-resistant infections

Staphylococcus aureus

Mycobacterium tuberculosis

Plasmodiumfalciparum

Streptococcus pneumoniae

methicillinmultidrug

any drugany drugmultidrug

chloroquine

penicillin

USJapan

USNew York CityEastern Europe

Kenya GhanaZimbabweThailand

US, Asia, ChileSpainHungary

3260

131620

65455945

10–352058



Glossary


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Which disease?


Multiple-choice quiz

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