#1) A few tens of thousand base-pairs length of DNA (or RNA) with a layer ofproteins wrapped around its outside (and maybe also a layer of membrane wrapped around that!)

Suppose that the proteins have some mechanism to get the DNA into cells of some bacteria, or some animal plant. (usually species specific)

Suppose that this DNA gets copied in the host cells;
and suppose that the DNA gets transcribed to make m-RNA
and the host cell ribosomes and t-RNA are used to make the proteins that the DNA codes for,

And that some of these proteins form a layer around the DNA copies, and the copies + proteins get released by the host cells.

That would be the life cycle of a typical virus.

#2) Many diseases are caused by viruses: examples include: influenza, measles, smallpox, AIDS, colds

#3) Viral diseases are not treatable by antibiotics;
(but there are anti-viral drugs; analogs to purines & pyrimidines;
but unfortunately none of these are very effective)

The immune system, and its stimulation by vaccines, is our main defense against viruses.

#4) Some viruses use single stranded DNA as their genetic material
other viruses use double stranded DNA,
Other viruses use RNA, in some kinds single-stranded and in other kinds double-stranded RNA.

In one group of viruses, the original virus has RNA, but also produces enzymes that make DNA with the same base sequence. "Reverse transcriptase" is the name of this enzyme. The AIDS virus does this.

#5) All viruses have protein capsids: some also have membrane envelopes around the outside.
the shapes of capsids are (almost always) either helical or icosahedral

#6) Some virus DNA sometimes gets inserted into the chromosomes of the host (animal or bacteria);
stays there for many generations, and then gets cut out again and resumes life as an infective organism. "lysogeny" in bacterial viruses.

#7) How do viruses evolve?: two theories; both might be true.

a) Highly reduced parasites (further evolution of organisms like Rickettsiae)

b) Escaped, run-away genes. like transposons, plasmids

new stains of Influenza: ducks -> pigs -> humans

Evolution within one species tends toward doing less harm to host.

#9) There are species of virus in bacteria, animals and plants.
Any given species typically has hundreds of different viruses,
many of which do them very little harm.

#10) in sea water, etc. there seem to be many more virus particles than any other (living) things.