  Viruses are incapable of autonomous energy production. Although many
experimental studies make it clear that viruses are parasitic entities that
hijack the host's molecular resources, a detailed estimate for the energetic
cost of viral synthesis is largely lacking. To quantify the energetic cost of
viruses to their hosts, we enumerated the costs associated with two very
distinct but representative DNA and RNA viruses, namely T4 and influenza. We
found that for these viruses, translation of viral proteins is the most
energetically expensive process. Interestingly, the cost of building a T4 phage
and a single influenza virus are nearly the same. Due to influenza's higher
burst size, however, the overall cost of a T4 phage infection is only 2-3% of
the cost of an influenza infection. The costs of these infections relative to
their host's estimated energy budget during the infection reveal that a T4
infection consumes about a third of its host's energy budget, where as an
influenza infection consumes only 1%. Building on our estimates for T4, we show
how the energetic costs of double-stranded DNA viruses scale with virus size,
revealing that the dominant cost of building a virus can switch from
translation to genome replication above a critical virus size. Lastly, using
our predictions for the energetic cost of viruses, we provide estimates for the
strengths of selection and genetic drift acting on newly incorporated genetic
elements in viral genomes, under conditions of energy limitation.
