Strain (biology)
A strain was a genetic variant or subtype of a microorganism.

In 2366, Paul Stubbs compared the nanite infestation of the computer core of the  to watching a strain the Leutscher virus reproduce. 
()

In a fantasy created by Barash's neural scanners on Alpha Onias III in 2367, William T. Riker was told he had been infected with a strain of Altarian encephalitis that had supposedly lain dormant within before becoming active sixteen years later. ()

Later that year, Jean-Luc Picard speculated that Beverly Crusher may have had a fast-growth strain of Diomedian scarlet moss to explain its showing one day of growth in less time than had apparently elapsed. ()

In 2368, Crusher wondered if a new strain of Iresine Syndrome had developed, to explain the normal histamine count in the comatose Deanna Troi and Riker. ()

In 2370, Mora Pol asked Jadzia Dax if an organic strain taken from an infected Odo matched silicate-based lifeforms from L-S VI. () 

In an imaginary scenario experienced by Kathryn Janeway in 2373, Janeway was informed by The Doctor that she had contracted a virulent form of the Vidiian phage that was spreading rapidly through her system. ()

In 2374, The Doctor claimed an injured Romulan's blood contained a strain of the Terothka virus as a ruse to get onto the bridge of the hijacked . ()

In 2375, The Doctor informed "DaiMon Torrot" – in fact Seven of Nine, whose own personality had been suppressed by those she’d assimilated while a Borg drone – that "he" had contracted a nasty strain of the Ankaran flu. ()

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Biology