HIV, which only infects humans, is considered to have a host range, whereas rabies virus, which can infect a number of animals and humans, is considered to have a host range.

narrow, broad

Explanation:

Host range in epidemiology refers to the number of potential host species a pathogenic organism is capable of infecting provided all other conditions are met. A pathogen with several potential host is said to have a wide host range while the one with very few potential host is said to have a narrow host range.

Hence, HIV in this case is considered to have a narrow host range while rabies virus is considered to have a wide host range.

not necessarily. higher fitness requires the organism to be reproductive successful. for example, an organism at age 10 with 15 offspring is much more fit than an organism at age 15 with only 5 offspring.