Influenza can be transmitted prior to symptom onset. A

mutation extends the duration of pre‑symptomatic viral shedding without

increasing immune detection or compromising replication.

Across many transmission events, what population‑level

effect is most likely?

Influenza viruses undergo antigenic drift via small

mutations in antibody‑binding regions of haemagglutinin. These changes reduce

neutralisation by vaccine‑induced antibodies while leaving

receptor binding intact.

If vaccination coverage remains high over many seasons, what

evolutionary outcome is most likely?

A viral variant shows enhanced replication in the lower

lungs, causing severe pneumonia, but reduced replication in the upper

respiratory tract. Hosts infected with this variant transmit the virus less

effectively before becoming severely ill.

Given sustained circulation attempts in humans, what outcome

is most likely?

Avian influenza viruses preferentially bind α2,3‑linked

sialic acid receptors, whereas human influenza viruses preferentially bind α2,6‑linked receptors found in the upper respiratory tract. In

the scenario where a mutation arises that enhances α2,6‑receptor

binding, increasing upper airway replication and person‑to‑person

transmission of Avian influenza. There is no reduction in viral fitness is

observed due to this mutation.

Given continued circulation in humans, what outcome is most

likely over time?

Seasonal influenza viruses encounter partial population

immunity, whereas pandemic strains emerge in immunologically naïve populations.

A new influenza variant appears with antigenic features poorly recognised by

existing antibodies.

Over multiple epidemic waves, what effect is this most

likely to have?

Most human influenza strains require lung‑specific

proteases to cleave hemagglutinin (HA). A novel mutation allows HA cleavage by

a broader range of host proteases, increasing infectivity without affecting

transmission efficiency or immune recognition.

Assuming this mutation does not increase host mortality

prior to transmission, what is the most likely long‑term outcome?

A hospital reduces empirical antibiotic prescribing,

shortens treatment durations, and favours narrow‑spectrum agents. Over several

years, rates of antibiotic resistance decline despite continued treatment of

infections.

Which change in selection pressure best explains this

outcome?

A mutation causes constitutive overexpression of a multidrug

efflux pump, conferring reduced susceptibility to several antibiotic classes.

The mutation carries a modest metabolic cost but provides survival advantage

during antibiotic exposure.

Carbapenem resistance appears suddenly across multiple

bacterial species in different hospitals and countries within a short time

frame. Genetic analysis reveals the same resistance gene on mobile plasmids in

each isolate.

A bacterial infection is treated repeatedly with the same

antibiotic. Each episode resolves clinically, but relapse occurs after

treatment cessation. The causative strain shows no increase in MIC and no

detectable resistance mutations. Further investigation reveals a persistent

subpopulation that survives antibiotic exposure through metabolic inactivity.

Assuming repeated antibiotic exposure over time, what type

of selection pressure is most likely acting on the bacterial population?