[li]Either:[/li][ul][li]You’ve got a strain of flu that you’ve had before, so your body’s defenses are already primed[/li][li]Your health and defenses are generally good, so they’re stronger from the start of your sickness[/li][li]You’ve still got the flu, but you’re either asymptomatic or the symptoms aren’t very noticable[/li][/ul]
[li]Arguably yes. The fever is one of the ways your body uses to kill “germs”. It’s more effective against bacteria, IIRC, but viruses also fall to it.[/li]
But if your fever gets too high (your body keeps detecting more and more viruses, but the fever isn’t killing them off faster than they multiply), your body cells start getting killed. So a slight fever is good, a high fever is bad.
[li]HIV is a fairly fragile virus as viruses go. Outside of a body and/or bodily fluids, they die quickly. But in body-body or fluid-fluid transferrs, they survive to multiply in the new host.[/li]
So on toilet seats, the virus will die, but in syringes they’ll live.
And as to its transmitability: think of it in terms of Star Trek: HIV is an attacking alien, the Enterprise’s shields are your body’s level of defense against HIV getting a foothold.
After the first attack (first exposure to HIV), the E’s shields are down to 75%. Very survivable.
If there’s a second attack before shields are back to 100%, the shields go down to 50%.
Following attacks take the shields down to 25%, the 0%. (At this point, there’s no eliminating the virus, and it begins to multiply.)
[li]Yes, it’s harder, though not impossible. An infected female’s lubricating fluids have HIV in it. The virus can then enter through the end of the urethra, or if the male has any abrasions or sores on his penis, through these.[/li]
Male-to-partner infections are more likely to take, as more fluid is left in the partner, so the virus thrives. And if there’s blood vessels broken during coitus, the infection is aided even more.