Pathogens and parasites Notes

What Is A Pathogen?

1. Pathogens harm hosts by disrupting normal body functions. They:
   1. Damage cells or tissues.
   2. Divert host resources for their own growth.
   3. Trigger symptoms that reduce host survival or reproduction.
2. Pathogens rely on hosts to survive and spread.
3. This means they can't complete their life processes independently, which makes successful transmission to new hosts essential.

What Are The Main Types Of Pathogens?

1. Bacteria
   1. Single-celled living organisms.
   2. Reproduce rapidly using host resources.
   3. Some produce toxins that damage host cells.
2. Viruses
   1. Non-living particles made of DNA or RNA inside a protein coat.
   2. Can only reproduce inside host cells.
   3. Often kill host cells when new viruses are released.
3. Fungi
   1. Eukaryotic organisms that often grow on host surfaces.
   2. Feed by digesting host tissues externally.
   3. Can invade deeper tissues if untreated.
4. Protists
   1. Single-celled eukaryotes.
   2. Often parasitic and adapted to life inside hosts.

How Do Pathogens Enter And Spread Between Hosts?

1. Pathogens must be transmitted between hosts to persist.
   1. Transmission routes include direct contact, body fluids, food, water, and air.
   2. Some pathogens require vectors.
2. A vector is an organism that transmits a pathogen between hosts.
   1. Mosquitoes are an example of a vector for malaria parasites.
   2. The mosquito itself isn't harmed, but the human host is.

How Do Different Pathogens Infect Hosts?

Different pathogens harm hosts in distinct ways.

Bacterial Infection

1. Bacteria grow and reproduce inside the host.
   1. They consume host nutrients.
   2. This reduces energy available to host cells.
2. Some bacteria release toxins.
3. Toxins damage tissues or disrupt nerve signals.
4. Symptoms are often caused by toxins, not the bacteria themselves.

Viral Infection

1. Viruses invade host cells.
   1. Viral genetic material enters the cell.
   2. The cell’s machinery is hijacked to produce new viruses.
   3. The host cell is usually destroyed.

Fungal Infection

1. Fungi spread using spores.
   1. Spores land on suitable host tissue.
   2. The fungus grows and digests host cells externally.
2. Surface infections can spread deeper if untreated.

Protist Infection

1. Protists live and feed inside the host.
   1. Many have complex life cycles.
   2. Some require multiple hosts to complete development.

How Do Pathogens Avoid The Immune System?

1. Some pathogens hide inside host cells.
   1. This protects them from immune attack.
2. Some change surface proteins (antigen variation).
   1. The immune system can no longer recognise them.
   2. This prolongs infection.
3. Some remain dormant.
   1. They reactivate when the immune system weakens.

What Is A Parasite?

1. Parasites depend on hosts for survival.
   1. They obtain food, shelter, or nutrients.
   2. They reduce host fitness rather than killing immediately.
2. Parasites often evolve to keep the host alive.
   1. This is because killing the host removes their resource supply.

What Are The Main Types Of Parasites?

1. Protist parasites
   1. Single-celled eukaryotes.
   2. Often live in blood or tissues.
2. Helminths (parasitic worms)
   1. Multicellular organisms.
   2. Attach inside the host using hooks or suckers.
3. Ectoparasites
   1. Live on the host’s surface.
   2. Feed on blood or skin.

How Are Parasites Adapted For Infection?

1. Attachment structures: Hooks or suckers prevent removal.
2. High reproductive output: Large numbers of eggs increase transmission success.
3. Complex life cycles: Multiple hosts increase spread opportunities.
4. Immune evasion strategies: Surface protein changes reduce detection.

How Are Pathogens And Parasites Different?

Some parasites are pathogens, but not all pathogens are parasites.

Why Do Some Parasites Control Host Behaviour?

1. Behavioral manipulation increases transmission, as hosts become more likely to encounter the next host.
2. This improves parasite reproductive success.

How Do Pathogens And Parasites Fit Into Ecosystem Interactions?

1. They regulate population size.
   1. Disease can reduce population density.
   2. Parasites can limit reproduction.
2. They are part of interdependent networks.
   1. Changes in host populations affect pathogen survival.
   2. Changes in pathogen pressure affect host populations.