Viruses

• Viruses are the smallest living organism   10nm-300nm
• they are v.hard to term dead or alive
• All viruses have a protein coat (capsid) that surrounds the nucleic acid 
• the nucleic acid can be RNA or DNA
• there are many types of viruses including: HIV and bacteriophages

Viruses only replicate within a host cell (and thus destroying the host)

• A bacteriophage (phage) is a virus that has a DNA core
• a phage attacks bacterial cells.
• They contain DNA bounded by a protein coat

• Phages invade the bacteria, inwhich they inject viral DNA 
• DNA replicates and is packed into protein coats 
• bacterial cells destroyed (as new phages released)

structure - HIV consists of a core of RNA bounded by a protein coat and a lipid bilayer containing glycoprotein.

HIV is retrovirus - 'any group of RNA viruses which insert a DNA copy of their                                        genome into the host cell in order to replicate.'

process

• HIV viruse deliver the RNA with the enzyme reverse transcriptase into the host cell.
• The enzyme catalyses the synthesis of DNA from RNA
• host cell makes copies of viruses

HIV invades a type of lymphocyte (T-helper cells) thus weakening the immune system.

how?

HIV destroys T-cells thus damaging the body's ability to fight off invading germs and diseases. The weakened immune system is known as AIDs.

note:

• AIDs is the result of an HIV infection, but not every1 with HIV has AIDs.
• people with HIV don't show symtoms