How are STI and STD spread? If two people have not had any sexual contact with any other person can an STI or STD be spread through anal or oral sex?

Mutual monogamy. Mutual monogamy, assuming both parties are negative for STD's, usually guarantee absence of transmission. The exception is previous exposure to Herpes which has not had any "outbreaks" over a period of time until resuming contact with another partner.
Person to person. STDs are spread from person to person in various ways. Some through transmission of fluids (gonorrhea, chlamydia, trichomoniasis, HIV). Others are spread through contact (Herpes, syphilis, HPV). One of you would have to have an STD to give it to the other. If neither person has ever had any sexual contact of any kind then they can not spread and STD.

Related Questions

Girlfriend and I both virgins, we never had any sexual contact of any kind (anal, oral, intercourse, etc) before. She just gave me oral, can I get an STD?

Possibly. Sexually transmitted diseases can be acquired through oral sex and can be transmitted through oral sex. If a person gave oral sex to an individual with a STD she can pass the STD on to the next person she gives oral sex to. Read more...

Do std's spread through oral contact/saliva (no direct sexual contact)?

Sometimes. If the infecting organism gets to the mouth by contact with an infected area one one person, another person could get the infection by kissing, but if is unusual. Read more...
Depends on STD. Oral contact can spread many bacterial stds period. This is especially true with a person with an active infection open sore hidden in the back of the mouth. Hiv doesn't as there is something in the saliva that kills it. Research is still being done to discover why. Read more...