1. Any of a group of RNA viruses whose RNA is used as a template inside a host cell for the formation of DNA by means of the enzyme reverse transcriptase. The DNA thus formed is inserted into the host cell's genome. Most retroviruses can cause cancer. Retroviruses also include HIV.
2. Prokaryotes are 1-celled organisms that do not have a nucleus or any other membrane-bound organelles inside them. The name prokaryote itself actually lets you know that there isn't a nucleus, since pro means before and karyo refers to nucleus.
3. Antibiotic resistance occurs when bacteria change in some way that reduces or eliminates the effectiveness of drugs, chemicals, or other agents designed to cure or prevent infections. The bacteria survive and continue to multiply causing more harm. Bacteria can do this through several mechanisms.
