Infectious Diseases in Humans

• Pathogen: A disease-causing organism
• Disease: A condition that causes the body to function less effectively. They produce specific signs or symptoms.

  • A sign of disease is visible (e.g. fever, coughing)
  • A symptom of disease is invisible (e.g. headache, fatigue)
• Infectious/transmissible disease: Disease caused by pathogens that can be passed from one person to another.

  • Caused by pathogens like bacteria and viruses. They can be spread through body fluids, food and water.
  • Influenza, HIV, pneumococcal diseases, malaria.
• Non-infectious disease: Disease not caused by pathogens and cannot be spread.

  • Inherited - diabetes, sickle shape anaemia
  • Malnutrition, environmental factors (pollution)
  • Liver cirrhosis (lifestyle choice: excessive alcohol consumption)
  • Coronary heart disease (lifestyle choice: excessive consumption of polyunsaturated fats and trans fats)
• Spreading of Infections Diseases

  • Passing of a pathogen to an uninfected person: transmission
  • Entry of the pathogen into the body: infection
  • Person where the pathogen lives and breeds: host
• Droplets in the air through respiratory passages

  • Tiny respiratory droplets are expelled when a person coughs/sneezes, and its secretions may contain pathogens
  • Anyone within a close range may breathe in these droplets and become infected
  • Distance/masks can reduce infection
• Direct contact

  • Direct person-to-person contact (e.g. exchanging body fluids during sexual intercourse)
  • Spread when the skin or mucous membrane of an infected person comes into contact with that of another person
  • STIs like Human Immunodeficiency Virus (HIV) can be transmitted through sexual intercourse
• Contamination

  • Food and water contaminated with pathogens (e.g. cholera and salmonella bacteria) can enter the alimentary canal
  • Occurs when food and water are not properly stored/handled
  • E. coli can be passed from raw meat to cooked meat if people have poor hygiene practices
• Bacteria

  • Does not have a membrane-bound nucleus
  • Has a singular circular DNA as its genetic material
  • Has organelles (cell wall, cell membrane, ribosomes), plasmids and one or more flagellum (tail)
  • Can be pathogenic and non-pathogenic
  • Can reproduce by itself
• Virus

  • Has a protein coat enclosing its genetic material (either DNA or RNA) - not membrane bound
  • No cellular structures (e.g. ribosomes)
  • Does not grow, move, feed, respire or excrete
  • Requires a host cell to reproduce and obtain its protein coat
• Signs and Symptoms of Influenza

  • Sore throat
  • Chills
  • Runny nose
  • Muscle ache
  • Fatigue
  • Fever
  • Headache
  • Cough
• Signs and Symptoms of Pneumococcal Diseases

  • Shortness of breath
  • Vomiting
  • Photophobia (light sensitivity)
  • Fever
  • Headache
  • Cough
• Ways to reduce transmission

  • Get the respective vaccination (annually for influenza)
  • Avoid close contact with infected people
  • Cover your mouth and nose when coughing/sneezing when sick and dispose of it after
  • Wash your hands with soap and water/disinfectant
  • Avoid touching eyes, nose and mouth
  • Wear a surgical mask
  • Take prescribed antiviral drugs (influenza)
  • Take prescribed antibiotics (pneumococcal)
• Vaccination

  • Contains an agent that resembles a harmless form of pathogen (weakened or dead) being introduced into the body by injection or swallowing
  • Presence of pathogens triggers WBC to quickly produce antibodies to combat possible infection
• How vaccinations work

  • Pathogens are recognised by the WBCs that lock onto their antigens if they enter the body later - memory cells are WBCs that remain in the bloodstream for a long time
  • Phagocytosis takes place more easily - antibodies clump the bacteria together or neutralise the toxins the bacteria produce
• Passive immunity

  • Short-term defence against a pathogen by antibodies acquired from another individual (e.g. mother to infant)
• Antibodies

  • Neutralise toxins produced by bacteria
  • Binds to bacteria and ruptures its surface membrane (antigens)
  • Agglutination thus increases the ease of phagocytosis
• Active immunity

  • Defence against a pathogen by antibody production in the body
  • Gained by vaccination if they have made their own antibodies and memory cells that protect against the pathogen - last for many years
• Examples of Vaccines

  • Harmless form of the pathogen - BCG inoculation against TB
  • Killed pathogen - Salk anti-polio vaccine
  • Toxoid (inactivated bacteria toxin)
• Antibiotics

  • Drugs used to treat bacterial infections
  • Target bacteria by preventing synthesis of cellular structures
  • Inhibits synthesis of bacterial cell wall (water can enter, cell expands, bursts and dies)
  • Breaks up cell membrane, cannot filter substances
  • Bind to ribosomes and prevent them from taking part in protein synthesis
  • Inhibits the enzyme needed for the synthesis of folic acid in bacterial cytoplasm
  • Can be chemically modified to increase effectiveness
• Why antibiotics can don't work on viruses

  • Antibiotics interfere with the growth and metabolic activities of bacterial cells
  • Viruses have different structures and reproductive components from bacteria (no cell walls, cell membranes, ribosomes, and requires a host cell)
• Antibiotic Resistance ("superbugs")

  • Genes may mutate when exposed to certain chemicals, mutagenic agents (e.g. radiation)
  • Bacteria less sensitive to an antibiotic will not be killed easily and survive other bacteria - reproduce and pass these advantageous alleles to the next generation
  • Results in more resistant bacteria
  • Further prescription of the same antibiotic will not kill them
  • Disease cannot be cured unless new antibiotics are discovered
• Reduction of Antibiotic Resistance

  • Not misusing or overusing antibiotics to treat a viral infection
  • Completing the course of the antibiotics prescribed
  • Using antibiotics only when necessary and not on minor infections