A virus's surface spike proteins recognise and bind a specific
host-cell receptor. After binding, the virus injects its genetic
material into the cell, hijacking host machinery to copy itself.
New virus particles then assemble and bud off to infect further
cells.
The full cycle runs through four stages: approach (the
virion drifts toward a host cell), bind (spike proteins
lock onto a matching receptor), inject (the genome is
delivered into the cytoplasm) and replicate (hijacked host
machinery mass-produces and buds off new virions). The immune
system counters this by dispatching immune cells that patrol the
tissue, recognise infected material and neutralise free virions
before they can infect further cells - slowing the rise in
virion count even while replication continues.
- Stage buttons - jump directly to approach, bind, inject
or replicate to inspect each step of the cycle.
- Infection rate - scales how quickly new virions bud
off the host cell during the replicate stage.
- Immune strength - controls how many immune cells are
deployed and how fast they hunt down and clear virions.
In real infections, antiviral drugs are designed to block a
specific stage - e.g. entry inhibitors prevent binding, while
polymerase inhibitors block replication - and how quickly a
person's immune response ramps up is a major factor in whether
an infection stays mild or becomes severe.