How does a virus reach its host, it is always passive?
There are a number of different ways to spread a viral infection. Some viruses sit on surfaces (or infected persons) "waiting" for someone to touch and take them up, this is called smear infection. Other viruses are airborn inside of small droplets distributed by coughing or sneezing. These are inhaled and can then cause a new infection. Other viruses are transmitted through our body fluids (blood, semen) and either need a wound or can be transmitted sexually (for example Herpes or HIV). Other viruses are transmitted by animals which bite humans (Dengue and rabies would be examples here). Have a look into the Wikipedia for more details and examples.

Additionally viruses are produced in vast numbers by their hosts with most viral particles never reaching a new host. This strategy helps a lot to transmit further.s

For entering the nucleus (which is an obstacle) the viruses use the nuclear pore complex (NPC). Depending on what virus tries to enter the nucleus, different strategies are used. Some viruses are too big and release their DNA through the NPC into the nucleus, while others can pass through and some get stuck inside and release their DNA/RNA at this point. A last group uses the time during mitosis when there is no nucleus. See this figure (from here)
and also the references below:



References:


Virus strategies for passing the nuclear envelope barrier.
How viruses access the nucleus.
Viruses challenge selectivity barrier of nuclear pores.