Opportunistic infections are infections that occur more frequently and are more severe in people with weakened immune systems, including people with HIV.
I think verterbrates have a back bone others do not. I only THINK so.
The Silurian period occurred before the Neogene period.
I hope this helps.
Answer:Capillaries
Explanation: Capillaries are tiny, thin walled vessels that form a network to take blood through the organs and other body tissues. The dense networks of capillaries present a large surface area, which allows materials to be exchanged between body cells and the blood rapidly.