Parasitism Essentials

Major metabolic saving: nutrition & resources obtained from host at ~ $0$ energetic cost
Energy diverted almost exclusively to replication/reproduction
Present across nearly all taxa → proven evolutionary strategy
Degree of host dependence varies widely among pro- & eukaryotic parasites
Viruses represent extreme: entirely reliant on host for replication machinery & building blocks

Reproduction tightly linked to host availability & signals ➔ host can control parasite development
Narrow host range if specific host cues required
Absence of suitable host = extinction risk
Survival outside host necessitates dormant/robust forms (viral particles, bacterial spores, protozoan cysts, helminth eggs) or prolific replication

Likely appeared early & evolved multiple times independently
Endosymbiotic theory: mitochondria & chloroplasts arose from engulfed aerobic & photosynthetic bacteria that became intracellular symbionts/parasites
- Step 1: membrane infoldings → nucleus & ER
- Step 2: engulf aerobic bacterium → mitochondrion
- Step 3: engulf photosynthetic bacterium → chloroplast
Most present-day cellular parasites descended from free-living ancestors subjected to selective pressures favoring parasitic traits

Two main hypotheses
- Reductive (regressive): degenerated descendants of cellular organisms ➔ evolved after first cells
- Primordial (virus-first): ancient, primitively non-cellular entities that co-evolved with earliest cells
Current data insufficient to confirm either scenario

Parasitism acts as strong selective pressure on host inflammatory & immune systems
Ongoing "arms race": host defenses improve → parasites evolve counter-measures (immune evasion, resistance)
Human behavior/technology also influences battle (e.g., antibiotics → selection for resistant bacteria)