B2. KEEPING HEALTHY

• feelings or changes you experience in your body when you are ill

  symptoms
• a disease caused by an infection that can be passed from one organism to another

  communicable disease
• diseases are caused by:

  virus, bacteria, fungi, protist
• diseases that are caused by a person's lifestyle, environment, or genes

  non-communicable disease
• single-celled prokaryotic organisms

  bacteria
• most bacteria do not cause human diseases; ex: bacteria in the gut help digest food and protect you against harmful bacteria
• fungi and protists are eukaryotic organisms
• most fungi are multi-cellular, but some are single-celled; ex: yeast
• protists can be single-celled or multi-cellular
• bacteria, fungi, and protists can reproduce whenever in warm conditions with plenty of nutrients, moisture, and space
• made of a genetic material wrapped in a layer of protein and sometimes fat molecules

  virus particle
• viruses can only reproduce by invading living cells
• a bacterium, fungus, protist, or virus that causes a disease

  pathogen
• each bacterium divides to produce two new ones and can divide once every 20 minutes
• bacteria can't keep dividing. when nutrients and space start to run out they limit the rate of reproduction
• the bacteria's own waste products can eventually poison and kill them
• bacteria in a sealed container can't keep reproducing because nutrients and space run out
• pathogens can cause symptoms of disease when they damage our cells by invading and reproducing, and by releasing toxins
• symptoms may only appear after a pathogen has divided enough times to produce a larger number of pathogens in the body
• the delay between infection and appearance of symptoms

  incubation period
• communicable diseases spread when pathogens are passed from an infected organism to a healthy one
• pathogens can be passed through:

  • bodily fluids (blood, saliva, mucus, semen)
  • waste products
  • contaminated food and water
  • by touch
• flu

  • pathogen: influenza virus
  • spread by: body fluids (cough and sneezes)
  • infects when: droplets are inhaled or transferred from hand to nose, mouth, or eye
• salmonella food poisoning

  • pathogen: salmonella bacterium
  • spread by: contaminated food and water
  • infects when: contaminated food and water are ingested
• athlete's foot

  • pathogen: fungus
  • spread by: surfaces
  • infects when: skin touches a contaminated surface
• malaria

  • pathogen: plasmodium protists
  • spread by: mosquitoes
  • infects when: mosquito bite introduces infected saliva into the blood
• diseases that are spread by unprotected sex or genital contact

  sexually transmitted infection (STI)
• examples of STIs:

  • chlamydia and gonorrhoea: caused by bacteria
  • genital warts: caused by viruses
• an STI that damages the body's ability to fight off other infections and is caused by the human immunodeficiency virus (HIV); spreads in semen and blood and has a long incubation period

  acquired immune deficiency syndrome (AIDS)
• crown gall disease

  • pathogen: bacterium
  • spread by: contaminated soil, water, and farming tools
  • infects: grapes, stone fruit, nut trees, sugar beet
  • one of the most serious plant diseases
• ash dieback

  • pathogen: fungus
  • spread by: spores carried by wind and movement of contaminated plant material
  • infects: ash trees
• tobacco mosaic virus

  • pathogen: virus
  • spread by: direct contact and contaminated seeds
  • infects: tobacco, tomato, bell pepper, cucumber, some flowers
  • first virus discovered in plants; studied as a model organism
• three types of human defences:

  • physical
  • chemical
  • bacterial
• prevents pathogens from invading the tissues; skin, and mucus in the respiratory and digestive systems, platelets

  physical defences
• destroys pathogens and stops them from reproducing; stomach acid, antimicrobial substances, enzymes in saliva, mucus, tears

  chemical defences
• competes against pathogens by using up nutrients and space; helpful bacteria that live on the skin and in the gut

  bacterial defences
• defences that are always present and are not made in response to a specific pathogen

  non-specific defences
• physical barrier that stops pathogens from entering the body

  skin
• fragments of cells made from the cytoplasm of large cells that help seal wounds

  platelets
• platelets stick to the cut edge and send out substances that trigger a series of reactions which makes the blood clot at the cut site.