Like Brian said, a virus doesn't conform to the standard definition of a living creature - no reproduction (apart from a host of an entirely different nature), no metabolism, no respiration (this has broad meaning and is sometimes lumped together with metabolism - don't think of this as only being identical with a human or a fish breathing), no response to stimuli (this can be rather broad in scope and meaning). So, the subject line of this Post is, admittedly, in one sense, a misnomer.
Viruses (or Virii, if you prefer - both are correct) enter cells through virus receptor sites on the cell. The protein sheath of the virus is able to afix itself to a somewhat complementary virus receptor site and injects its DNA or RNA (a virus has one or the other, not both) into the host cell. Some viruses also are encapsulated in a membrane which is often similar to a host cell's membrane or membranes of a host's cell's intracellular organelles.
Inside the host cell is where it takes over and splices the invading virus DNA or RNA into the host cell's DNA, and forces the host cell to replicate copies of the complete virus particle - nucleic acid core (DNA or RNA) and the protein sheath. Though some viruses may remain dormant for some time and not begin this replication cycle which results in destruction of the host cell.
When a sufficient number of virus copies are replicated, known as the "burst size", the cell can't hold anymore and ruptures, releasing the virus particles into the environment (that environment varies widely, it could be the blood stream, water, etc). For reference, the burst size for the T5 bacteriophage which infects the E. coli bacterium is ~200. The virus particles at this point are NOT inside of any cell, but for a time are free floating/roaming hoping to contact an acceptable host cell with an appropriate virus receptor site that is complementary, so to speak, to a portion of the virus's protein sheath, and the cycle begins all over again.
When free from the confines of a host cell, the virus particle is very susceptible to destruction from ionized chemicals, UV light, electro-static discharge, etc. Often, (always???) they are somewhat protected by other substances with which it may be associated such as mucus, droplet nuclei, etc.
Viruses range in sizes from 20 or so nanometers up to a bit over 200 nanometers, IIRC (but you should verify this as my memory is not too good anymore). In fact, there is actually some overlap in sizes b/t the smallest virus particles and the largest proteins! I've never come across the pore size of an expanded condom, so, i'll take your word for it.
Does this answer the very astute question that you posed?