Infectious diseases are mostly caused by bacteria and viruses.


  • Useful bacteria are found on the skin and in the digestive tract
  • Prokaryotic cellular structure, single celled organelles
  • Prokaryotic cells store carbon compounds in the form of glycogen and lipids.
  • Ribosomes – site of protein synthesis, smaller than in eukaryotic cells, occur free in the cytoplasm
  • DNA is circular not chromosome form, attached to cell membrane at least at one point
  • Plasmid – small circles of DNA containing non-essential genes. Can be exchanged between different bacterial cells
  • Cell membrane – made of phospholipids and proteins
  • Mesosome – tightly-folded (infolded) region of the cell membrane containing proteins required for respiration and photosynthesis
  • Cell wall – made of protein/peptidoglycan, does not contain celluluse
  • Have cell surface membrane, cytoplasm, cell wall, ribosomes, plasmids, sometimes mesosomes, flagellum and pili
  • Pili – protein tubes that allow bacteria to attach to surfaces
  • Flagellum – used for cell movement
  • Capsule – a mucus layer for protection and allow bacteria to form colonies
  • They can live independently

Gram stain – used to colour the bacterial cell wall for identification:

Gram positive bacteria – have a thick cell wall and stain purple. Teichoic acid bind to crystal violet.

Gram negative bacteria – have a thin cell wall with an outer lipid layer and stain pink. Don’t have teichoic acid, crystal violet is decolourised, replaced by safranine


Bacterial reproduction

Binary fission – replication of DNA, replication of plasmids, cytoplasm and cell wall splits into two

Generation time = time between divisions of bacteria

Transformation – many can acquire new genes by taking up DNA molecules from their surroundings

Transduction – bacterial DNA is moved between bacteria by a virus

Conjugation – some bacteria can transfer their chromosome to a recipient bacteria



  • Smallest living organism
  • No cell wall, cell surface membrane, organelles or cytoplasm
  • Obligate parasites – cannot reproduce without a host. Invade other cells and take over their biochemistry to make more viruses
  • Nucleic acid core (DNA or RNA), enclosed in a protein coat (capsid) – made of protein subunits called capsomeres
  • Some have lipid rich covering around capsid called the envelope which is usually formed from host cell membrane and may have spikes to help recognise and attach to the host cell. They contain glycoproteins from the virus – these are antigens, recognised by the immune system as non self
  • Retroviruses are viruses with RNA as their genetic material
  • Slime layer (capsule) – polysaccharide layer outside of the cell wall. Used for sticking cells together, food reserve, protection
  • Pathogenic bacteria – cause infectious diseases e.g. TB, cholera
  • Wide range of shapes and sizes
  • After reproducing inside the host cells, new virus particles may bud from the cell surface or burst out of the cell – killing the cell – lysis



  • Caused by the bacterium mycobacterium Tuberculosis
  • Caused by inhalation – droplet infection. Close contact, poor health, poor diet and overcrowded living conditions increase the risk
  • Treatment: prolonged drug treatment (3-9 months), at least 3 antibiotics used. Bacteria destroyed before they cause damage. Rest, healthy diet, observation therapy
  • Prevention: improve living standards, treat cattle diseases, protective clothing
  • There are two phases of the disease:
  • Primary infection – may have no symptoms. immune system responds through an inflammatory response. In a healthy person macrophages engulf the bacteria but tubercules form. TB can survive inside macrophages, resisting killing mechanisms by having a thick waxy cell wall which makes them difficult to break and hides their antigens from other macrophages. They can be dormant for years and suppress T cells, reducing antibody production and attach killer cells.
  • Secondary infection – If the immune system can’t contain the diseases or an infection may break out if the immune system is weak (old age, malnutrition or poor living conditions)


  • Affects the lungs – Tuburcules – a mass of tissue formed in the lung as a result of inflammatory response
  • infection occurs through inhaling contaminated air or drinking infected milk
  • primary infection – early stage of infection
  • development of tubercle
  • bacteria are destroyed by WBC and the tissue heals
  • some bacteria produce thick waxy layer around them and survive
  • they remain dormant but if the person’s immune system is weakened these bacteria become active and divide
  • this leads to active TB – lung tissue is slowly destroyed by the bacteria causing symptoms of: breathing problems, coughing up blood in sputum, suppression of immunity, weight loss, poor appetite, fever and sweats
  • the TB bacteria also targets cells of the immune system so the patient cannot fight other infections
  • HIV causes AIDS which directly targets white blood cells, reducing a patient’s ability to fight infection
  • The initial symptoms include fever, general weakness and severe coughing, caused by inflammation of the lungs. As TB progresses, it damages the lungs and can lead to respiratory failure and death. TB can spread from the lungs to other parts of the body and can lead to organ failure.


90% of those infected with M. tuberculosis never develop TB because:

  • They are immune/resistant/vaccinated
  • Have antibodies/memory cells
  • Bacteria destroyed before they cause damage



  • Causes AIDS
  • Symptoms of aids – fever, diarrhoea, weight loss, secondary infections such as TB/pneumonia , fatigue,
  • 3 – 12 months after infection, HIV antibodies appear in the blood so the person is HIV positive
  • AIDS usually develops after 8 – 10 years
  • Transmission – sexual contact, infected blood (drug users sharing needles/repeated use of needles) or from mother to foetus (early stages of pregnancy, during birth or breastfeeding)
  • Prevention – condoms, clean needles and awareness programmes


HIV replication:

  • HIV attaches to surface of macrophage (host cell)
  • Gp120 protein (virus surface) and CD4 protein (host cell membrane)
  • The enzyme reverse transcriptase makes a DNA copy of the viral RNA
  • DNA copy is replicated by the same enzyme
  • This new DNA moves into the nucleus and integrated into the host cell chromosome by the enzyme integrase
  • Using the host cell machinery, mRNA is synthesised from the new proviral RNA
  • Viral mRNA is translated to make viral enzymes and structural proteins
  • Viral RNA genome is also made from proviral DNA
  • The viral genome and structural proteins assemble to form the basic structure of the virus
  • These move out of the host cell by exocytosis, taking part of the cell membrane with it (lipid layer around the virus)
  • Eventually the gene that codes for gp120 protein is mutated
  • The new protein attaches to a different CD4 protein present in T cells
  • The same cycle repeats in T cells
  • But as the virus leaves the T cell, it destroys the cell membrane killing the host cell – this reduces the number of T cells in the body reducing immunity


Difficult to create a vaccine for AIDS because:

  • virus mutates rapidly
  • therefore antigens on the viral surface continually change
  • working on animals to develop vaccines not possible because HIV only infects humans
  • the virus hides itself for years inside macrophages, therefore most may not work properly


  1. Explain why unbroken skin is an effective barrier against HIV infection:
  • keratin/protein in skin surface/epidermis
  • forms a hard/impenetrable/physical barrier


  1. Explain the change in numbers of CD4 T-lymphocytes during the first 6 weeks after infection with HIV:
  • glycoprotein/gp120 on virus
  • binds with receptors/CD4
  • on surface membrane of lymphocytes
  • viral RNA enters the lymphocyte
  • viral RNA used to produce viral DNA in lymphocyte
  • by action of reverse transcriptase
  • formation of new viruses
  • lymphocyte destroyed when new viruses bud out of/leave cell
  • T killer cells/lymphocytes destroy T helper cells/lymphocytes


  1. Suggest one effect that this change would have on one other component of the infected blood:
  • B cells not activated/are inhibited
  • Fewer antibodies/T killer cells increase


Evolutionary race – TB and HIV

The bacterium which causes TB and the virus which causes AIDS have evolved features to help them evade the immune system.

  • TB – a thick waxy coat is produced which protects it from the enzymes of the macrophages
  • TB – when engulfed by phagocytes, they produce substances that prevent the lysosome fusing with the vacuole – aren’t broken down and so can multiply undetected in phagocytes
  • Disrupts antigen presentation in infected cells – preventing immune system from recognising and killing infected phagocytes
  • HIV – the protein coat is constantly changing – the immune system cannot target and destroy it
  • HIV – reduces the number of immune cells in the body – reduces the change of HIV being detected
  • HIV – high rate of mutation in the genes which code for antigen proteins – change the structure of antigens – antigenic variation. The memory cells won’t recognise other strains with different antigens – has to produce a primary response for each new strain


The carbohydrate present in storage granules is glycogen.